<PAGE>
                                                                    Exhibit 10.4

                                  AMENDMENT TO

                        UNITED HEALTHCARE SERVICES, INC.

                                       AND

                           UNITEDHEALTH NETWORKS, INC.

                      PHARMACY BENEFIT MANAGEMENT AGREEMENT

         This Amendment is made to the agreement ("Agreement") between United
HealthCare Services, Inc. and UnitedHealth Networks, Inc., collectively referred
to as "United", and the entity named below ("MMMC"). The Agreement currently
sets forth the terms and conditions under which MMMC or an affiliate shall
provide or arrange for the provision of health care services to individuals
covered by a United's affiliated Health Plan pursuant to its Medicare contract
with the Health Care Financing Administration ("HCFA"). The parties understand
and agree that the Balanced Budget Act of 1997 ("BBA") established a new program
known as Medicare+Choice, which replaced Health Plan's existing Medicare risk
program. The purpose of this Amendment is to incorporate all provisions
necessary to meet the HCFA requirements for Medicare+Choice. This Amendment is
effective on August 1, 1999.

1.   The Agreement shall be amended by the addition of the attached Health Plan
     Medicare+Choice Requirements Addendum.

2.   All other provisions of the Agreement shall remain in full force and
     effect.


UNITED HEALTHCARE SERVICES, INC.      MERCK MEDCO MANAGED CARE, L.L.C.

Signature  Illegible                  Signature  Illegible
           --------------------                  -----------------------

Title      President                  Title      Senior Vice President-
                                                 Regulatory and Managed
                                                 Care Programs

Date       1/29/01                    Date       1/24/01


UNITEDHEALTH NETWORKS, INC.

Signature  /s/ Illegible
           --------------------

Title      Secretary

Date       1/26/01

<PAGE>

                                   HEALTH PLAN

                      MEDICARE+CHOICE REQUIREMENTS ADDENDUM

In addition to PBM's obligations under the Agreement, PBM agrees, and shall
require PBM Contracting Providers to agree, as participating providers under
aUnited affiliated Health Plan's contract with HCFA to be a Medicare+Choice
managed care organization (hereafter the "M divided by C Contract"), to abide by
all applicable provisions of the M+C Contract and to fulfill PBM's and PBM
Contracting Provider's obligations under the Agreement in a manner consistent
with a United affiliated Health Plan's (hereafter the "Health Plan") obligations
under the M divided by C Contract. For purposes of this Addendum, "Medicare
Member" means a Health Plan's Member who is enrolled in a Medicare+Choice plan
through Health Plan. PBM and PBM Contracting Provider compliance with the M+C
Contract specifically includes, but is not limited to, the following
requirements:

1.   Prompt Payment. Health Plan shall pay "clean" claims for Covered Services
     within forty- five (45) days of receipt and approve or deny all claims that
     are not "clean" claims within sixty (60) days from the date of the request.

2.   Medicare Compliance. PBM shall, and shall require PBM Contracting Providers
     to, comply with all applicable Medicare laws and regulations and HCFA
     instructions.

3.   Audits and Information. In addition to PBM's and PBM Contracting Provider's
     obligations under Section 5.3.2, PBM shall, and shall require PBM
     Contracting Providers to, permit audits and inspection by HCFA and/or its
     designees, and cooperate, assist and provide information to HCFA and/or its
     designees as requested from time to time. This provision shall survive
     termination of the Agreement.

4.   Maintenance of Records. In addition to PBM's and PBM Contracting Providers'
     obligations under Section 5.3.2, PBM shall, and shall require PBM
     Contracting Providers to, retain books, contracts, documents, papers and
     records, including without limitation, medical records, patient care
     documentation, and other records that pertain to any aspect of services
     performed, financial solvency, reconciliation of benefit liabilities and
     determination of amounts payable under Health Plan's M divided by C
     Contract for a minimum of six (6) years from the end of the applicable
     one-year contract period in the M divided by C Contract or the completion
     of an audit, or in certain instances described in applicable
     Medicare+Choice regulations, for periods in excess of six (6) years, if
     appropriate. PBM shall, and shall require PBM Contracting Providers to,
     maintain such records accurately and update them on a regular basis. PBM
     and PBM's employees and agents shall, and shall require PBM Contracting
     Providers to, maintain the confidentiality of all Medicare Member records
     in accordance with the applicable laws and regulations, and shall safeguard
     Medicare Members' privacy. This provision shall survive termination of the
     Agreement.

5.   Data Collection. PBM shall submit to Health Plan, upon request, all data
     necessary for Health Plan to fulfill its reporting obligations pursuant to
     42 C.F.R.SS.422.516. PBM

<PAGE>

     must submit to Health Plan all data, including medical records, necessary
     to characterize the content and purpose of each encounter with a Medicare
     Member. PBM must certify (based on best knowledge, information and belief)
     the accuracy, completeness and truthfulness of such data on certification
     forms provided by Health Plan. PBM shall hold harmless and indemnify Health
     Plan for any fines or penalties it may incur due to PBM's submission of
     inaccurate or incomplete data.

6.   Accountability. PBM acknowledges, and shall require PBM Contracting
     Providers to acknowledge, that Health Plan oversees and is responsible to
     HCFA for any functions or responsibilities provided or performed by PBM or
     PBM Contracting Providers pursuant to the M+C Contract, as applicable.

7.   Delegation. If any service or activity to be performed by PBM under this
     Agreement is delegated, to the extent permitted by and in accordance with
     this Agreement, to a downstream entity, such entity shall enter into a
     contract with PBM obligating such entity to perform such service or
     activity consistent with and in compliance with the terms of this Agreement
     and the M+C Contract.

8.   Continued Care. In addition to PBM's and PBM Contracting Providers'
     obligations pursuant to Section 3.8 of the Agreement, PBM shall, and shall
     require PBM Contracting Providers to, provide Covered Services to Medicare
     Members (i) for all Medicare Members, for the duration of the M+C Contract
     period for which HCFA payments have been made; and (ii) for Medicare
     Members who are hospitalized on the date the M+C Contract terminates or in
     the event of Health Plan's or PBM's insolvency, through discharge. This
     provision shall survive termination of the Agreement.

9.   Compliance with Pharmacy Services Manual. PBM shall require PBM Contracting
     Providers to comply with PMB's Pharmacy Services Manual, including, without
     limitation, the Medicare Plus Choice Requirements addendum (the "Pharmacy
     Attachment"). PBM shall comply with those requirements of the Pharmacy
     Attachment applicable to the performance of PBM's obligations under this
     Agreement, including, without limitation, where an obligation is placed
     upon PBM Contracting Providers but such obligation may be performed or
     could be violated by PBM. In the event of a conflict between any provision
     in this ADDENDUM and the Pharmacy Attachment, this ADDENDUM shall govern.

<PAGE>

                                  AMENDMENT TO

                        UNITED HEALTHCARE SERVICES, INC.

                      PHARMACY BENEFIT MANAGEMENT AGREEMENT

         This Amendment is made to the Pharmacy Benefit Management Agreement
("Agreement") between United HealthCare Services, Inc., UnitedHealth Networks,
Inc. (collectively "United") and Merck-Medco Managed Care, L.L.C. ("PBM") dated
November 11, 1998.


WHEREAS, the Agreement sets forth the obligations of the parties in order for
United to make available pharmacy benefit management and related services to
Health Plans and other non-Health Plan business and PBM agreed to provide such
services; and

WHEREAS, the parties desire to amend the Agreement in order to add PAID
Prescriptions, L.L.C. ("PAID") as a signatory to the Agreement.

NOW THEREFORE, in consideration of the terms and conditions set forth in this

Addendum, the parties agree as follows:

1. PAID is a subsidiary of PBM and acts as a Third Party Administrator ("TPA")
for PBM on behalf of United. PAID is licensed in certain states as a TPA as
required by applicable law.

2. The parties agree that PAID is added as a signatory to the Agreement shall
perform the TPA functions in the Agreement.

3. All other provisions of the Agreement shall remain in full force and effect.


United HealthCare Services, Inc.             Marck-Medco Managed Care, L.L.C.

Signature: Illegible                         Signature: Illegible
           --------------------                         -------------------

Title:     COO                               Title:     Vice President

Date:      3/28/01                           Date:      4/12/01



United HealthNetworks, Inc.                  PAID Prescriptions, L.L.C.

Signature: Illegible                         Signature: Illegible
           --------------------                         -------------------

Title:     Vice President                    Title:    Vice President

Date:      3/28/01                           Date:     4/12/01



<PAGE>

                                                                    Exhibit 10.4

Letter Agreement
----------------

United HealthCare Services, Inc.
9900 Bren Road East,
P.O. Box 1459 Minnetonka, MN 55343
Attention: William A. Munsell

          Re:  Pharmacy Benefit Management Agreement (the "Agreement") between
               United HealthCare Services, Inc., on behalf of itself and its
               affiliates from time to time, ("United HealthCare") and
               Merck-Medco Managed Care, LLC, ("PBM").

          United HealthCare and PBM agree to make the following changes to the
Agreement:

          1. ***

          2. ***

          3. Rebate Contracting.

             (a)  PBM agrees to work with United HealthCare to negotiate new
                  or amended rebate agreements with drug manufacturers by June
                  30, 2002 that will result in United HealthCare earning ***

             (b)  ***

             (c)  ***

             (d)  ***

          4. ***

          5. Effect of this Letter Agreement.

               Except as specifically modified by this Letter Agreement, the
          Agreement and the letter agreement between the parties dated
          September 9, 1998 shall remain in effect.

          6. Defined Terms.

               Except as otherwise defined in this Letter Agreement, capitalized
          terms shall have the meanings set forth in the Agreement.

<PAGE>

          Except as otherwise defined in this Letter Agreement, capitalized
          terms shall have the meanings set forth in the Agreement.

ACCEPTED AND AGREED:


UNITED HEALTHCARE                           MERCK-MEDCO MANAGED
SERVICES, INC.                              CARE, LLC.


BY:     /s/ William A. Munsell              BY:    /s/ Glenn Taylor
       ---------------------------------           -----------------------------
           (signature)                                (signature)

NAME:  William A. Munsell                   NAME:  Glenn Taylor

TITLE: President, United Health Networks    TITLE: President -
                                                   United Health Group Division

DATE:  June 29, 2001                        DATE:  June 29, 2001



                                    EXHIBIT A

***



***  Represents text deleted pursuant to a confidentiality treatment request
     filed with the Securities and Exchange Commission.

<PAGE>

                                    AMENDMENT

United HealthCare Services, Inc. ("United HealthCare") and Merck-Medco Managed
Care, LLC ("PBM") hereby agree to amend that certain letter agreement, executed
June 29, 2001 by and between the parties, a copy of which is attached to this
amendment as Attachment I (the "Letter Agreement"), as follows:

     1. ***

     2. ***

     3. ***

     4. ***

     5. Except as otherwise defined in this Amendment, capitalized terms shall
        have the meanings set forth in the Pharmacy Benefit Management Agreement
        between United HealthCare and PBM that was executed by United HealthCare
        on November 11, 1998 (the "Agreement"). Except as specifically modified
        by this Amendment, the Agreement and the Letter Agreement shall remain
        in effect.

ACCEPTED AND AGREED as of the 26th day of October 2001.

United HealthCare Services, Inc.               Merck-Medco Managed Care, LLC


By:  Illegible                                 By: Glenn Taylor
     ---------                                     ------------
Title: Vice President                          Title: Sr. Vice President
       --------------                                -------------------

                              EXHIBIT A RESTATEMENT
                                OCTOBER 26, 2001

***




*** Represents text deleted pursuant to a confidentiality treatment request
filed with the Securities and Exchange Commission.

<PAGE>

                                    AMENDMENT

United HealthCare Services, Inc., on behalf of itself and its affiliates from
time to time (collectively, "United HealthCare") and Merck-Medco Managed Care,
LLC ("PBM") hereby agree to make the following changes to the Pharmacy Benefit
Management Agreement between United HealthCare and PBM that was executed by
United HealthCare on November 11, 1998 (the "Agreement"):

     1. Section 4.6 of the Agreement is hereby revised to read as follows:

        "4.6 Minimum PDL Enrollment. Effective as of December 11, 2001 and
        continuing throughout the term of this Agreement, United HealthCare
        agrees that it shall maintain a minimum of *** Covered Persons
        (including, as a subset thereof, at least *** non-Health Plan Covered
        Persons) receiving services under this Agreement including
        participating in United HealthCare's PDL. ***

     2. Section 3.15 is hereby deleted from the Agreement.

     3. Except as otherwise defined in this Amendment, capitalized terms shall
        have the meanings set forth in the Agreement. Except as specifically
        modified by this Amendment, the Agreement as heretofore amended shall
        remain in effect.

ACCEPTED AND AGREED as of the 19th day of December 2001.

United HealthCare Services, Inc.              Merck-Medco Managed Care, LLC

By:    /s/ William A. Munsell                 By:    /s/ Glenn Taylor
       -------------------------                     --------------------------

Name:  William A. Munsell                     Name:  Glenn Taylor

Title: Vice President                         Title: Sr. Vice President




*** Represents text deleted pursuant to a confidentiality treatment request
filed with the Securities and Exchange Commission.