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As filed with the Securities and Exchange Commission on April 20, 2006

Registration No. 333-128913

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

AMENDMENT NO. 4

TO

FORM S-1

REGISTRATION STATEMENT

UNDER

THE SECURITIES ACT OF 1933

Eastern Insurance Holdings, Inc.

(Exact name of registrant as specified in its articles of incorporation)

25 Race Avenue

Lancaster, Pennsylvania 17603

(717) 396-7095

(Address, including zip code, and telephone number, including area code, of registrant’s principal executive offices)

(Name, address, including zip code, and telephone number, including area code, of agent for service)

Copies to:

Approximate date of commencement of proposed sale to the public: As soon as practicable after this Registration Statement becomes effective.

If any of the securities being registered on this form are to be offered on a delayed or continuous basis pursuant to Rule 415 under the Securities Act of 1933, check the following box.  x

If this form is filed to register additional securities for an offering pursuant to Rule 462(b) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.  ¨

If this form is a post-effective amendment filed pursuant to Rule 462(c) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.  ¨

If this form is a post-effective amendment filed pursuant to Rule 462(d) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier registration statement for the same offering.  ¨

If delivery of the prospectus is expected to be made pursuant to Rule 434, check the following box.  ¨

The registrant hereby amends this registration statement on such date or dates as may be necessary to delay its effective date until the registrant shall file a further amendment which specifically states that this registration statement shall thereafter become effective in accordance with Section 8(a) of the Securities Act of 1933 or until the registration statement shall become effective on such date as the Commission, acting pursuant to said Section 8(a), may determine.

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The information in this prospectus is not complete and may be changed. We may not sell these securities until the registration statement filed with the Securities and Exchange Commission is effective. This prospectus is not an offer to sell these securities and it is not soliciting an offer to buy these securities in any state where the offer or sale is not permitted.

Subject to Completion, dated                     , 2006

PROSPECTUS

EASTERN INSURANCE HOLDINGS, INC.

We are offering up to 8,305,556 shares of our common stock for sale in connection with the conversion of Educators Mutual Life Insurance Company, or Educators, from the mutual to the stock form of organization. Immediately following the conversion, we will acquire all of the newly issued shares of Educators’ common stock and will become the holding company for Educators. We are offering shares of our common stock in a subscription offering simultaneously to persons in the following listed categories, and subscriptions will be filled in the following order of priority:

The subscription offering will end at noon, Eastern Time, on May     , 2006. Any shares of our common stock not sold in the subscription offering may be offered in a community offering, which will end concurrently with the subscription offering unless extended by us. We refer to the subscription offering and the community offering as the conversion offering. The ESOP will purchase 10% of the total number of shares sold in the conversion offering. See “The Conversion and the Merger.”

Immediately after the completion of the conversion offering, we will acquire EHC for a purchase price of approximately $78.9 million and will merge EHC with and into a recently formed wholly owned subsidiary. In the merger, we will issue to EHC shareholders 3,875,472 shares of our common stock and pay approximately $40.2 million in cash in exchange for all outstanding shares of EHC stock. This prospectus covers the shares of our common stock offered in the conversion offering. Shares issued in the merger will be in addition to the shares sold in the conversion offering.

EHC, an insurance holding company, is not affiliated with Educators or Eastern Holdings, and if the merger and the conversion are not completed, they will remain unaffiliated. Neither the conversion nor the merger will be completed unless both are completed.

A minimum of 5,525,000 shares of common stock must be sold in the conversion offering to complete the conversion. We may sell between 5,525,000 and 7,475,000 shares without resoliciting subscribers. Our ESOP will purchase 10% of the total number of shares sold in the conversion offering. Therefore, the maximum number of shares sold may be increased to 8,305,556 shares solely to accommodate the 10% interest being purchased by our ESOP. Shares issued to the ESOP will be counted, but shares issued in the merger will not be counted, toward satisfaction of the minimum amount. If more subscriptions are received than shares offered, shares will be allocated among eligible subscribers in the manner and priority described in this prospectus. See “The Offering.”

The minimum number of shares that a person may subscribe to purchase is 25 shares. The maximum number of shares that a person may subscribe to purchase is 100,000 shares, and the maximum number of shares that any person and his or her affiliates, or any group acting in concert, may purchase is 150,000 shares. For further information regarding the limitations on purchases of common stock in the conversion offering, see “The Offering—Limitations on Common Stock Purchases.” Once submitted, orders are irrevocable unless we terminate the conversion offering or extend the offering beyond                     , 2006. Funds received prior to completion of the conversion offering will be held in an escrow account at Christiana Corporate Services, Inc. If the conversion offering is terminated prior to completion, subscribers will have their funds returned promptly, without interest.

Keefe, Bruyette & Woods, Inc. will use its best efforts to assist us in selling our common stock in the conversion offering, but is not obligated to purchase any shares of common stock that are being offered for sale. Subscribers will not pay any commission to purchase shares of common stock in the conversion offering.

There is currently no public market for our common stock. Keefe, Bruyette & Woods, Inc. has advised us that it intends to make a market in our common stock, but it is under no obligation to do so. Our common stock has been approved for quotation on The NASDAQ National Market under the symbol “EIHI.”

This investment involves risk. For a discussion of the material risks that you should consider, see “ Risk Factors” beginning on page 10 of this prospectus.

OFFERING SUMMARY

Price: $10.00 Per Share

Neither the Securities and Exchange Commission, the Pennsylvania Insurance Department, nor any state securities commission has approved or disapproved of these securities or determined if this prospectus is truthful or complete. Any representation to the contrary is a criminal offense.

For assistance, please contact the Stock Information Center at (888) 427-7355.

KEEFE, BRUYETTE & WOODS

The date of this prospectus is April     , 2006

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CERTAIN IMPORTANT INFORMATION

You should rely only on the information contained in this prospectus. We have not, and Keefe, Bruyette & Woods has not, authorized any person to provide you with information that is different from that contained in this prospectus. If anyone provides you with different or inconsistent information, you should not rely on it. We and Keefe, Bruyette & Woods are offering to sell and seeking offers to buy our common stock only in jurisdictions where offers and sales are permitted. You should assume that the information contained in this prospectus is accurate only as of the date of this prospectus, regardless of the time of delivery of this prospectus or of any sale of our common stock. Our business, financial condition, results of operations and prospects may have changed since that date. In this prospectus:

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PROSPECTUS SUMMARY

This summary highlights selected information from this prospectus and may not contain all of the information that is important to you. To understand the conversion offering and the merger fully, you should read this entire prospectus carefully, including the financial statements and the notes to financial statements of Educators and EHC included in this prospectus.

Eastern Insurance Holdings, Inc.

We are a Pennsylvania corporation organized by Educators and EHC. Following the mutual-to-stock conversion of Educators and the merger of our wholly owned subsidiary with EHC, we will be the holding company for Educators and EHC. We are not an operating company and have not engaged in any business to date. Our executive offices are located at 25 Race Avenue, Lancaster, Pennsylvania 17603, and our telephone number is (717) 396-7095. We have not yet established a website.

Educators Mutual Life Insurance Company

Educators is a Pennsylvania-domiciled mutual life and health insurance company that offers group benefits products. Originally founded in 1910 to offer disability income products to public school teachers in central Pennsylvania, Educators offers group dental, disability and life insurance products. Educators’ target policyholder base is primarily composed of small and medium size employers of 300 or fewer employees. Educators previously offered group medical insurance, but terminated the sale of these products in 2002 due to competitive market conditions.

Educators actively markets its group benefits products primarily in the Mid-Atlantic, Southeast and Midwest regions of the continental United States, and is licensed to do business in 41 states and the District of Columbia. Educators offers its products through independent insurance agents, general agencies and brokers, which we refer to as producers. With the exception of IBSi, formerly a wholly owned subsidiary of Educators, for the year ended December 31, 2005, none of Educators’ independent producers represented 5% or more of its direct premiums written. Direct premiums written excludes the impact of all reinsurance premiums, either assumed or ceded. See “Business of Educators.”

At December 31, 2005, Educators had total consolidated assets of $111.2 million and total equity of $62.1 million. For the years ended December 31, 2005 and 2004, Educators had direct premiums written of $41.0 million and $40.8 million, respectively, and consolidated net income of $1.1 million and $1.8 million, respectively.

Educators’ executive offices are located at 202 North Prince Street, Lancaster, Pennsylvania 17603 and its telephone number is (717) 397-2751. Educators’ website address is www.emlife.com.

Eastern Holding Company, Ltd.

EHC is a Cayman Islands holding company incorporated in 1997. EHC’s principal subsidiaries are Global Alliance Holdings Ltd., doing business as Eastern Alliance Insurance Group, a Pennsylvania corporation, and Eastern Re, a Cayman Islands corporation. Global Alliance has three operating subsidiaries—Eastern Alliance Insurance Company, Allied Eastern Indemnity Company, and Employers Alliance, Inc. Eastern Alliance and Allied Eastern, which we collectively refer to as EAIG, are Pennsylvania stock property and casualty insurance companies; Employers Alliance, a Pennsylvania corporation, is a third-party claims administrator.

EHC provides a broad range of workers’ compensation products through Eastern Alliance and Allied Eastern, some of which are reinsured by Eastern Re. EHC also provides claims administration and risk management services through Employers Alliance. Through quota share assumed reinsurance agreements at Eastern Re, EHC also participates in programs covering underground storage tanks, non-hazardous waste haulers and fire sprinkler contractors, all of which are underwritten by other unaffiliated insurance companies. EHC manages these operations through four business segments: workers’ compensation insurance, specialty reinsurance, segregated portfolio cell reinsurance business, and corporate/third party administration. EHC’s primary market is small and medium size Pennsylvania employers generally with

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300 employees or less for whom service, in addition to price, is frequently a key determinant in the choice of insurer. See “Business of EHC.”

At December 31, 2005, EHC had total consolidated assets of $190.2 million and shareholders’ equity of $56.7 million. For the years ended December 31, 2005 and 2004, EHC had gross premiums written of $95.0 million and $83.2 million, respectively, and consolidated net income of $16.0 million and $6.4 million, respectively.

EHC’s headquarters is located at the Genesis Building, 5th Floor, Grand Cayman, Cayman Islands, B.W.I., and its telephone number is (345) 949-7966. The website address of its United States companies is www.eains.com.

Our Strategy

We will be the holding company for Educators and EHC after the conversion and the merger. After the conversion and the merger, we will offer a diverse range of insurance products, primarily to small and medium size businesses, through our operating subsidiaries. We will use the capital provided by the conversion offering primarily to expand EHC’s workers’ compensation and related businesses. We also plan to integrate Educators’ group benefits business with EHC’s workers’ compensation business. The key elements of our strategy are:

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Risks Associated with Our Strategy

Our ability to successfully execute this strategy is subject to a number of significant risks. The most significant of these risks include:

See “Risk Factors—Risk Factors Relating to Our Business.”

Conversion of Educators from Mutual to Stock Form

Educators is a mutual life and health insurance company. A mutual insurance company has no stockholders; it has policyholders who have contractual rights and certain limited statutory rights such as voting rights with respect to the election of directors and consideration of fundamental transactions, including the conversion of the company from mutual to stock form. A stock company is a company or corporation whose capital is divided into shares and is owned by shareholders who assume the risks of profit or loss.

Educators adopted a plan of conversion on March 17, 2005, as amended on June 9 and November 17, 2005. The conversion involves a series of transactions by which Educators will convert from a mutual life and health insurance company to a stock life and health insurance company. Following the conversion, Educators will become our subsidiary and will change its name to Eastern Life and Health Insurance Company. Educators will continue to be subject to the regulation and supervision of the Pennsylvania Insurance Department, which we refer to as the Department. See “The Conversion and the Merger.”

As part of the conversion, we are offering for sale in a subscription offering between $55.3 million and $74.8 million of our common stock. This amount may be automatically increased to $83.1 million solely to accommodate the purchase of a 10% interest by our ESOP if Educators’ members subscribe for more than 90% of the $74.8 million of common stock originally offered. The subscription offering is being made simultaneously to persons in the following listed categories, and the subscriptions will be filled in the following order of priority:

The per share purchase price in the conversion offering will be $10.00. All investors will pay the same price per share in the offering. The conversion offering is scheduled to end at noon, Eastern Time, on                     , 2006. If an insufficient number of shares are subscribed for in the subscription offering, we may sell shares in a community offering, with preference given to residents of Lancaster County, Pennsylvania.

The conversion will permit members of Educators; the directors, officers and employees of Educators and EHC; shareholders of EHC; and possibly others from the local community and the general public to become shareholders of Eastern Holdings and to share in its future. The conversion also will make possible the proposed acquisition of EHC and provide additional capital that will enhance the ability of Educators and EHC to expand their operations.

Completion of the conversion is subject to various conditions, including:

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The Merger

Contemporaneously with the adoption by Educators of the plan of conversion, Educators and EHC entered into a merger agreement dated March 17, 2005. The merger agreement was the result of arms’ length negotiations between Educators and EHC. The purchase price to be paid for EHC is $78.9 million.

Educators has received an opinion from Keefe, Bruyette & Woods, Inc. that the terms of the merger are fair to Educators from a financial point of view.

As a result of the merger, EHC shareholders will own at least 31.8% and as much as 41.2% of the outstanding shares of Eastern Holdings, depending upon the number of shares sold in the conversion offering. This percentage ownership interest will increase if EHC’s management and EHC shareholders also purchase shares of Eastern Holdings common stock in the conversion offering.

Completion of the merger is subject to various conditions, including:

Approval of both the domestication of EHC as a Pennsylvania corporation and the merger require the affirmative vote of two-thirds of the votes cast by EHC common and class A voting preferred shareholders voting as a single class at the special meeting of shareholders. In addition, the merger requires the affirmative vote of 75% of the votes cast by holders of EHC class A voting preferred stock at the special meeting of shareholders. In connection with the execution of the merger agreement, directors of EHC, who hold 12.0% of the issued and outstanding common and 48.0% of the issued and outstanding class A voting preferred shares of EHC, have agreed to vote their shares of EHC stock in favor of the merger and the domestication. Eastern Holdings, Educators, and EHC have received approvals of the plan of conversion and the merger agreement from the Department. The completion of the merger is expected to occur immediately after the completion of the conversion.

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Our Structure Following the Conversion and the Merger

The following chart shows our corporate structure following completion of the transactions described in this prospectus:

Directors and Management of Eastern Holdings

Our board of directors consists of eleven members, six of whom are directors of Educators and five of whom are directors of EHC. Mr. Robert M. McAlaine, our non-executive Chairman, also serves as the non-executive Chairman of Educators. Bruce M. Eckert, our Chief Executive Officer and a director, also serves as the Chief Executive Officer of EHC. Michael L. Boguski, our President and Chief Operating Officer, and Kevin M. Shook, our Treasurer and Chief Financial Officer, hold the same positions at EHC. See “Management.”

How We Determined the Price and the Number of Shares to be Issued in the Conversion Offering

Pennsylvania law requires that, as part of the conversion of Educators, we must sell an amount of our common stock in the conversion offering based on a value established by an independent appraiser. The value can be expressed as a valuation range. An independent appraisal of Educators’ pro forma market value has been prepared by Feldman Financial Advisors, Inc., an independent appraisal firm experienced in appraisals of insurance companies. The pro forma market value is the estimated market value of Educators as a wholly owned subsidiary of Eastern Holdings. In its opinion as of March 23, 2006, Feldman Financial estimated that the market value of our common stock being offered for sale was between $55.3 million and $74.8 million, with a midpoint of $65.0 million, and, based on the $10.00 per share purchase price, the number of shares being offered will range from 5,525,000 to 7,475,000. This amount may be automatically increased up to $83.1 million, and the number of shares being offered may be increased to 8,305,556 shares, solely to accommodate the purchase by our ESOP of 10% of the total number of shares sold in the conversion offering if Educators’ members subscribe for more than 90% of the $74.8 million of common stock originally offered.

The appraisal will be updated before the conversion is completed. If the midpoint of the updated appraisal is not within the valuation range, the revised appraisal will require the approval of the Department. If we are unable to sell at least 5,525,000 shares, then, unless the offering range is revised with Department approval, the conversion offering will be terminated, all subscriptions will be cancelled and all subscription funds returned, without interest. The current and proposed directors and executive officers of Educators, EHC and Eastern Holdings, as a group, have indicated their intention to

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subscribe for an aggregate of up to 358,750 shares of Eastern Holdings common stock. Purchases by management and purchases by our ESOP will be counted in determining whether sufficient subscriptions have been received to satisfy the minimum offering amount. See “The Offering—How the Price Per Share and the Offering Range Were Determined” for a description of the factors and assumptions used to determine the stock price and offering range.

The independent appraisal is an estimate of the pro forma market value of Educators as a subsidiary of Eastern Holdings. However, it is only an estimate of value; do not assume or expect that the valuation of Educators prepared by Feldman Financial means that our stock will trade at or above the $10.00 purchase price after the conversion.

Use of Proceeds from the Conversion Offering

We expect the net proceeds of the conversion offering to be between $51.2 million and $78.6 million, after the payment of our offering expenses. We intend to use the net proceeds from the conversion offering as follows (in thousands):

After paying our offering expenses, we will use the net proceeds received from the sale of common stock in the conversion offering to make a loan to our ESOP in an amount sufficient to permit the plan to buy up to 10% of the shares sold in the conversion offering.

In addition, we will use approximately $40.2 million of the net proceeds to pay the cash portion of the consideration in the merger. We will retain any remaining proceeds for general corporate purposes, which likely will include contributing additional capital to the domestic insurance company subsidiaries of EHC to support expansion of their underwriting capacity. Finally, assuming shareholder approval of a proposed stock compensation plan after the conversion is completed, we intend to purchase a number of shares equal to 4% of our common stock sold in the conversion offering to fund the restricted stock component of the stock compensation plan.

Except for the foregoing, we currently have no specific plans, intentions, arrangements or understandings regarding use of the proceeds of the offering.

The Amount of Stock That May Be Purchased in the Conversion Offering

The minimum number of shares a person may subscribe for in the conversion offering is 25 shares. Generally, subscribers in the subscription offering may purchase no more than 100,000 shares of common stock. The maximum amount of shares that a subscriber, together with any associate or any person with whom he or she is acting in concert, may purchase is 150,000 shares. For this purpose, an associate of a person includes:

We may decrease or increase the maximum purchase limitation without further notice. See “The Offering—Limitations on Common Stock Purchases.”

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How You Can Pay For Your Conversion Shares

Subscribers may pay for shares purchased in the conversion offering only by personal check, bank check or money order. Checks and money orders should be made payable to Christiana Corporate Services, Inc., as escrow agent.

Deadline for Orders of Conversion Stock

A properly completed stock order form, together with payment for the shares must be received by Educators no later than noon, Eastern Time, on                     , 2006, unless this deadline is extended by Educators to a date not later than                     , 2006. Subscribers may submit order forms by mail using the return envelope provided, by overnight courier to the indicated address on the order form, or by bringing their order forms to Educators’ offices during regular business hours. Once submitted, orders are irrevocable unless the offering is terminated or extended beyond                     , 2006.

Termination or Extension of the Conversion Offering

The subscription offering and community offering will expire at noon, Eastern Time, on                     , 2006. However, Educators may extend the expiration date for the community offering without notice, until                     , 2006. If the community offering extends beyond                     , 2006, we will distribute an amended prospectus to all subscribers, giving them the opportunity to confirm, modify or rescind their subscriptions.

Subscription Rights Are Not Transferable

Holders of subscription rights may not assign or sell their subscription rights. If a subscriber exercises subscription rights, he or she will be required to certify that the shares are being purchased solely for the subscriber’s own account and that there is no agreement or understanding regarding the sale or transfer of shares. We intend to pursue any and all legal and equitable remedies if we learn of the transfer of any subscription rights. We will reject orders that we determine involve the transfer of subscription rights. See “The Offering—Restrictions on Transfer of Subscription Rights and Shares.”

Our Dividend Policy

After the conversion is completed, our board of directors will have the authority to declare dividends on our common stock, subject to statutory and regulatory requirements. The Department’s order approving the conversion and the merger prohibits the declaration or payment of any cash dividend by Eastern Holdings, without the Department’s prior approval, for a period of three years after the effective date of the conversion. During this period, if we seek and obtain the Department’s approval, we intend to consider a policy of paying cash dividends on our common stock. The rate of such dividends and the initial or continued payment thereof will depend upon a number of factors, including the amount of net proceeds we retain in the conversion offering, investment opportunities available, capital requirements, our financial condition and results of operations, tax considerations, statutory and regulatory limitations, the amount of dividends that our insurance company subsidiaries can pay to us, and general economic conditions. We can provide no assurance that any dividends will be paid or that, if paid, will not be reduced or eliminated in future periods.

Interests of Directors and Officers in the Conversion and the Merger

Directors and officers of EHC who are shareholders of EHC may have economic interests in the conversion and the merger that are different from or in addition to, the interest of Educators policyholders. These interests include rights under employment agreements entered into contemporaneously with the plan of conversion and the merger agreement, rights to continued indemnification and insurance coverage by Eastern Holdings after the merger, and rights of EHC directors and officers under EHC’s stock option plan. In addition, directors and officers of EHC who are EHC shareholders will receive cash and shares of Eastern Holdings common stock in exchange for their shares of EHC in the merger. The following table sets forth, for each EHC officer and director, the fully diluted book value of his or her EHC shares as of December 31, 2005 and the cash and pro forma book value of the shares of Eastern Holdings common stock that each such officer and director will receive in the merger.

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Employees, officers, and directors of Eastern Holdings, Educators and EHC also will be able to acquire shares of Eastern Holdings common stock pursuant to various stock-based benefit plans either adopted in connection with the conversion offering or subsequent to its completion. Shareholders of Eastern Holdings who are not directors or employees of Eastern Holdings or one of its subsidiaries will not have an opportunity to participate in such plans.

Our ESOP will buy shares of common stock offered in the conversion offering with a portion of the net proceeds received in the offering. The shares will be allocated to the employee participants in the ESOP over a period of ten years at no cost to the employees.

The stock compensation plan will be implemented if we receive shareholder approval of the plan. Such approval cannot be obtained earlier than six months after the conversion. If the stock compensation plan is approved by our shareholders, we intend to grant restricted stock awards and stock options to certain officers, employees and directors. The restricted stock awards will consist of shares of Eastern Holdings’ common stock, which will be issued at no cost to directors and certain officers and employees and will vest over a period of time specified in the award in exchange for continued service. The stock options also will be issued to directors, officers and employees without cost to them, but they will be required to pay the applicable exercise price at the time of exercise to receive the shares of common stock covered by the options.

You will find more information about our ESOP and the stock compensation plan by reading the section of this prospectus entitled “Management—New Stock Benefit Plans.”

The following table summarizes the stock benefits that directors, officers and employees may receive in connection with or subsequent to the conversion offering, assuming that the number of shares sold is at the midpoint of the offering range:

In addition, five executive officers of EHC entered into employment agreements and two executive officers of Educators entered into retention agreements contemporaneously with the execution of the merger agreement.

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RISK FACTORS

You should carefully consider the risks described below before making an investment decision. If any of these risks actually occur, our business, financial condition and results of operations could be materially adversely affected and the price of our common stock and the value of your investment in our common stock could decline substantially. The use of the terms “we” and “our” throughout this “Risk Factors” section refers to Eastern Holdings, Educators and EHC collectively or individually as the context requires.

Risk Factors Relating to Our Business

Our results may be adversely affected if our actual losses exceed our loss reserves.

Both Educators and EHC maintain loss reserves to cover estimated amounts needed to pay for insured losses and for the loss adjustment expenses necessary to settle claims with respect to insured events that have occurred, including events that have not yet been reported to us. Estimating loss and loss adjustment expense reserves is a difficult and complex process involving many variables and subjective judgments; reserves do not represent an exact measure of liability. Accordingly, our loss reserves may prove to be inadequate to cover our actual losses. We regularly review our reserving techniques and our overall amount of reserves. We review historical data and consider the impact of various factors such as: