Document:

EXHIBIT 10.51

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                                FOURTH AMENDMENT

         This Fourth Amendment (the "4th Amendment") is made and entered into as
of this 27th day of December 1999 by and between NATIONAL DIAGNOSTICS, INC., an
Florida corporation ("NDI") and American Enterprise.com, Corp., formerly known
as American Enterprise Solutions, Inc., a Florida corporation ("AESI").

                                    RECITALS

         WHEREAS, NDI and AESI have entered into that certain Merger Agreement
dated February 23, 1998 as amended by that certain First Amendment dated March
17, 1998 and that certain Second Amendment dated April 29, 1998 and that Third
Amendment dated July 24, 1998 (the "Agreement") pursuant to which it is
contemplated with AESI will be merged (the "Merger") with and into NDI under the
terms and conditions specified in the Agreement; and

         WHEREAS, Section 4.1 of the Agreement currently provides, among other
things, that the Closing of the Merger shall occur on July 31, 1998, or as soon
as practicable after all conditions to Closing shall have been satisfied or
waived, or at such other time and date as NDI and AESI may mutually agree; and

         WHEREAS, AESI and NDI have mutually agreed that the date of Closing the
Merger shall be on or before December 31, 2000, or as soon as practicable after
all conditions to Closing shall have been satisfied or waived, or at such other
time and date as NDI and AESI may mutually agree; and

         WHEREAS, Section 13.1(ii) of the Agreement currently provides, among
other things, that AESI (acting through its board of directors) shall have the
right to terminate the Agreement if the Closing shall not have occurred by
December 31, 1998; and

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         WHEREAS, AESI and NDI have mutually agreed that Section 13.1(ii) of the
Agreement should be amended to reflect the fact that AESI (acting through its
board of directors) shall have the right to terminate the Agreement if the
Closing has not occurred by December 31, 2000.

         NOW, THEREFORE, in consideration of the premises set forth herein, and
for other good and valuable consideration, the receipt and sufficiency of which
are hereby acknowledged, the parties hereto, intending to be legally bound,
agree that the Agreement is hereby amended to incorporate and reflect the
following facts, terms and conditions:

         Section 4.1 of the Agreement is hereby amended to read as follows:

                 4.1    PLACE AND DATE OF CLOSING. Delivery of the stock
                        certificates referred to in Section 3 above, and
                        consummation of the other transactions contemplated by
                        this Agreement (hereinafter referred to as the
                        "Closing") shall take place at the offices of AESI, 6800
                        N. Dale Mabry, Suite 100, Tampa, Florida 33614 (or at
                        such other location as may be agreed upon by AESI and
                        NDI) on or before December 31, 2000, or as soon as
                        practicable after all conditions to Closing shall have
                        been satisfied or waived, or at such other time and date
                        as NDI and AESI may mutually agree, which date shall be
                        referred to as the "Closing Date."

         Section 13.1(ii) of the Agreement is hereby amended to read as follows:

                 (ii)   by AESI (acting through its board of directors) if the
                        transactions contemplated by this Agreement to take
                        place at the Closing shall not have been consummated by
                        December 31, 2000, unless the failure of such
                        transaction s to be consummated is due to the willful
                        failure of such transactions to be consummated is due to
                        the willful failure of AESI to perform any of its or his
                        obligations under this Agreement to the extent required
                        to be performed by it prior to or on the Closing Date:
                        or

         This Amendment may be executed in two or more counterparts, each of
which shall be deemed an original and all of which together shall constitute but
one and the same instrument.

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         IN WITNESS WHEREOF, the parties hereto have executed this Amendment as
of the day and year first written above.

                                          NATIONAL DIAGNOSTICS, INC.

                                          BY: /s/ CURTIS L. ALLISTON
                                              --------------------------------
                                          Name:  Curtis L. Alliston
                                          Title: President & Director

                                          AMERICAN ENTERPRISE.COM, CORP.

                                          BY:  /s/ CARDWELL C. NUCKOLS, PH.D.
                                               -------------------------------
                                          Name:  Cardwell C. Nuckols, Ph.D.
                                          Title: President & Director<PAGE>

                                                                     EXHIBIT 4.1

                                EMB CORPORATION
                                1999 STOCK PLAN

1.  Purpose.  The purpose of this 1999 Stock Plan (the "Plan") is to advance the
interests of EMB Corporation, a Hawaii corporation ("EMB"), and its shareholders
by offering to those officers, employees and directors of EMB and its
subsidiaries who will be responsible for the long-term growth of EMB's earnings
the opportunity to acquire or increase their equity interests in EMB, thereby
achieving a greater commonality of interest between shareholders, employees and
directors, enhancing EMB's ability to retain and attract both highly qualified
employees and directors and providing an additional incentive to such employees
to achieve EMB's long-term business plans and objectives.

2.  Award Opportunities.  Awards (individually, an "Award"; collectively, the
"Awards") under the Plan may be granted in the form of Common Stock that is
restricted and must be purchased by the officer, employee or director (the
"Restricted Common Stock") .

3.  Administration.

        (A) Committee. The Plan shall be administered by EMB's Compensation
        Committee (the "Committee") of the Board authorized by the Board. The
        Committee shall consist of no less than two directors of EMB who shall
        be appointed, from time to time, by the Board. All references in the
        Plan to the Board shall refer only to the Committee.

        (B) Authority. The Board, or the Committee, to the extent the Board has
        delegated such authority to the Committee, shall have full and final
        authority with respect to the Plan (i) to interpret all provisions of
        the Plan consistent with law; (ii) to determine the individuals who will
        receive Awards; (iii) to determine the frequency of grant of Awards;
        (iv) to determine the number of shares of Restricted Common Stock to be
        granted to each individual; (v) to prescribe the form and terms of
        instruments evidencing any Award granted under the Plan; (vi) to adopt,
        amend and rescind general and special rules and regulations for the
        Plan's administration; and (vii) to make all other determinations
        necessary or advisable for the administration of the Plan. The Board
        may, with the consent of the person who has been granted an Award under
        the Plan, amend the instrument regarding such Award consistent with the
        provisions of the Plan.

        (C) Indemnification. No member of the Board or the Committee shall be
        liable for any action taken or determination made in good faith. The
        members of the Board and the Committee shall be indemnified by EMB for
        any acts or omissions in connection with the Plan to the full extent
        permitted by Federal or California laws.

4.      Eligibility. Participation in the Plan shall be determined by the Board
        and shall be limited to officers, employees and directors of EMB and its
        subsidiaries (individually, a "Participant"; collectively, the
        "Participants").

5.      Stock Subject to Plan. Subject to adjustments as provided in Section
        9(A) hereof, the
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        aggregate amount of Common Stock as to which Awards may be granted under
        the Plan shall not exceed 1,700,000 shares and may be authorized but
        unissued shares or treasury shares.

        The Board shall maintain records showing the cumulative number of shares
        of Restricted Common Stock.

6.      Restricted Common Stock.

        (A) Granting of Restricted Common Stock. The Board may, in its sole
        discretion and subject to the provisions of the Plan, grant to eligible
        officers, employees or directors at such times as it deems appropriate
        following adoption of the Plan by the Board, shares of Restricted Common
        Stock.

        (B) Price of Restricted Common Stock. The price at which Restricted
        Common Stock may be purchased by a Participant under the Plan shall be
        determined by the Board and shall be 100% of the fair market value, per
        share, of the Common Stock at the time the Restricted Common Stock was
        granted.

            Each certificate evidencing shares of Restricted Common Stock shall
        be inscribed with a legend substantially as follows:

            "The shares of common stock of EMB Corporation, evidenced by this
            certificate are subject to the terms and restrictions of the EMB
            Corporation 1999 Stock Plan. Such shares are subject to forfeiture
            or cancellation under the terms of said Plan and shall not be sold,
            transferred, assigned, pledged, encumbered, or otherwise alienated
            or hypothecated except pursuant to the provisions of said Plan, a
            copy of which is available from EMB Corporation, upon request."

        (C) Persons Subject to Section 16 of the Exchange Act. Participants who
        are subject to Section 16 of the Exchange Act are hereby advised that
        reliance on Rule 16b3 may require that any equity security of EMB
        acquired upon exercise of Restricted Common Stock by such person be held
        at least until the date six months after the date of grant of the
        Restricted Common Stock.

7.      Other Provisions.

        (A) No Right to Employment. Neither the adoption of the Plan nor its
        operation, nor any document descrEMBng or referring to the Plan, or any
        part thereof, shall confer upon any Participant under the Plan any right
        to continue in the employ of EMB or a subsidiary or shall in any way
        affect the right and power of EMB or a subsidiary to terminate the
        employment of any Participant under the Plan at any time with or without
        assigning a reason therefor.
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        (B) Tax Withholding. The Board shall have the right to deduct from any
        settlement of an Award, including without limitation the delivery or
        vesting of Common Stock, made under the Plan any Federal, state, or
        local taxes of any kind required by law to be withheld with respect to
        such payments or to take any such other action as may be necessary in
        the opinion of the Board to satisfy all obligations for payment of such
        taxes. If Common Stock that would otherwise be delivered in settlement
        of the Award are used to satisfy tax withholding, such Common Stock
        shall be valued based on their Fair Market Value determined in
        accordance with section 6(B) when the tax withholding is required to be
        made. Participants who are subject to Section 16 of the Exchange Act are
        hereby advised that pursuant to Rule 16b3 thereunder the use of shares
        to satisfy tax withholding will be treated as the exercise of a Stock
        Appreciation Right.

        (C) Amendment and Termination. The Board may at any time suspend, amend,
        or terminate the Plan, and, without limiting the foregoing, the Board
        shall have the express authority to amend the Plan from time to time,
        with or without approval by the shareholders, in the manner and to the
        extent that the Board believes is necessary or appropriate in order to
        cause the Plan to conform to provisions of Rule 16b3 under the Exchange
        Act and any other rules under Section 16 of the Exchange Act, as any of
        such rules may be amended, supplemented, or superseded from time to
        time. Except for adjustments made in accordance with Section 9(A), the
        Board may not, without the consent of the grantee of the Award, alter or
        impair any Award previously granted under the Plan. No Award may be
        granted during any suspension of the Plan or after termination thereof.

        In addition to Board approval of an amendment, if the amendment would:
        (i) materially increase the benefits accruing to Participants; (ii)
        increase the number of shares of Common Stock deliverable under the Plan
        (other than in accordance with the provisions of Section 9(A); or (iii)
        materially modify the requirements as to eligibility for participation
        in the Plan, then such amendment shall be approved by the holders of a
        majority of EMB's outstanding capital stock represented and entitled to
        vote at a meeting held for the purpose of approving such amendment to
        the extent required by Rule 16b3 of the Exchange Act.

        (D) Effective Date of the Plan. The Plan was adopted by the Board on
        December 14, 1999.

        (E) Duration of the Plan. Unless previously terminated by the Board, the
        Plan shall terminate at the close of business on December 13, 2004, and
        no Award shall be granted under it thereafter, but such termination
        shall not affect any Award theretofore granted.

        (F) Use of Certain Terms. The terms "parent" and "subsidiary" shall have
        the meanings ascribed to them in Section 424 of the Code and unless the
        context otherwise requires, the other terms defined in Section 421, 422,
        and 424, inclusive, of the Code and regulations and revenue rulings
        applicable thereto, shall have the meanings attributed to them therein.

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