<PAGE>   1
                                                                       Exhibit 4

                                     364 DAY
                                CREDIT AGREEMENT

                         DATED AS OF SEPTEMBER 25, 1998,

                       AS AMENDED AS OF SEPTEMBER 24, 1999

                                      AMONG

                              HCR MANOR CARE, INC.,

                                MANOR CARE, INC.,

                     BANK OF AMERICA, NATIONAL ASSOCIATION,

                            AS ADMINISTRATIVE AGENT,

                            THE CHASE MANHATTAN BANK,

                              AS SYNDICATION AGENT,

                                DEUTSCHE BANK AG,

                             AS DOCUMENTATION AGENT,

                                       AND

                  THE OTHER FINANCIAL INSTITUTIONS PARTY HERETO

                                   ARRANGED BY

                        BANK OF AMERICA SECURITIES, LLC,

                                AS LEAD ARRANGER

                                       AND

                              CHASE SECURITIES INC.

                                       AND

                            DEUTSCHE BANC ALEX.BROWN,

                                 AS CO-ARRANGERS


                                       A-1

<PAGE>   2


                               FIRST AMENDMENT TO
                            364 DAY CREDIT AGREEMENT

         THIS FIRST AMENDMENT TO 364 DAY CREDIT AGREEMENT is made and dated as
of September 24, 1999 (the "FIRST AMENDMENT") among HCR MANOR CARE, INC., a
Delaware corporation (the "COMPANY"), MANOR CARE, INC., a Delaware corporation
("MANOR CARE"; Manor Care and the Company are collectively called the
"BORROWERS" and are each individually called a "BORROWER"), the financial
institution's party to the Credit Agreement referred to below, and BANK OF
AMERICA, NATIONAL ASSOCIATION, a national banking association, as Administrative
Agent (the "AGENT"), and amends that certain 364 Day Credit Agreement dated as
of September 25, 1998 (as amended or modified from time to time, the "CREDIT
AGREEMENT").

                                    RECITALS
                                    --------

         WHEREAS, the Borrowers have requested that the Agent and the Banks
amend certain provisions of the Credit Agreement, and the Agent and the Banks
are willing to do so, on the terms and conditions specified herein;

         NOW, THEREFORE, for good and valuable consideration, the receipt and
adequacy of which are hereby acknowledged, the parties hereby agree as follows:

         1. TERMS. All terms used herein shall have the same meanings as in the
Credit Agreement unless otherwise defined herein.

         2. AMENDMENTS. The Credit Agreement is hereby amended as follows:

                  2.1 AMENDMENTS TO COVER PAGE. The cover page of the Credit
Agreement is hereby amended and restated in its entirety to read as set forth in
Exhibit A hereof.

                  2.2 AMENDMENTS TO SECTION 1.1.

                  (a) The chart that appears in the definition of the term
"Applicable Facility Fee Rate" in Section 1.1 of the Credit Agreement is hereby
amended and restated in its entirety to read as follows:

        -----------------------------------------------------------------------
        Level        Leverage Ratio                               Applicable
        -----        --------------                            Facility Fee Rate
                                                               -----------------
        -----------------------------------------------------------------------

        Level 1      (less than) 1.50                                0.125%
        -----------------------------------------------------------------------

        Level 2      (greater than or equal to) 1.50
                         but (less than) 2.00                        0.150%
        -----------------------------------------------------------------------

<PAGE>   3


        -----------------------------------------------------------------------
        Level        Leverage Ratio                               Applicable
        -----        --------------                            Facility Fee Rate
                                                               -----------------
        -----------------------------------------------------------------------

        Level 3      (greater than or equal to) 2.00
                         but (less than) 2.50                        0.175%
        -----------------------------------------------------------------------

        Level 4      (greater than or equal to) 2.50
                         but (less than) 3.00                        0.200%
        -----------------------------------------------------------------------

        Level 5      (greater than or equal to) 3.00                 0.225%
        -----------------------------------------------------------------------

                  (b) The chart that appears in the definition of the term
"Applicable Margin" in Section 1.1 of the Credit Agreement is hereby amended and
restated in its entirety to read as follows:
<TABLE>
<CAPTION>

        ---------------------------------------------------------------------------------

        Level        Leverage Ratio                                   Applicable Offshore
        -----        --------------                                       Rate Margin
                                                                          -----------
        <S>          <C>                                                        <C>
        ---------------------------------------------------------------------------------

        Level 1      (less than) 1.50                                            +0.500%
        ---------------------------------------------------------------------------------

        Level 2      (greater than or equal to) 1.50 but (less than) 2.00        +0.600%
        ---------------------------------------------------------------------------------

        Level 3      (greater than or equal to) 2.00 but (less than) 2.50        +0.700%
        ---------------------------------------------------------------------------------

        Level 4      (greater than or equal to) 2.50 but (less than) 3.00        +0.925%
        ---------------------------------------------------------------------------------

        Level 5      (greater than or equal to) 3.00                             +1.275%
        ---------------------------------------------------------------------------------
</TABLE>


                  (c) There shall be added to Section 1.1 of the Credit
Agreement a new definition of the term "Applicable Utilization Fee Rate" reading
in its entirety as follows:

                  "APPLICABLE UTILIZATION FEE RATE" means a rate per annum
determined by reference to the Leverage Ratio as follows:
<TABLE>
<CAPTION>

        -------------------------------------------------------------------------------------------------
        Level            Leverage Ratio                                           Applicable Utilization
        -----            --------------                                                  Fee Rate
                                                                                         --------
        -------------------------------------------------------------------------------------------------
        <S>              <C>                                                      <C>
        Level 1          (less than) 1.50                                                 +0.100%
        -------------------------------------------------------------------------------------------------

        Level 2          (greater than or equal to) 1.50 but (less than) 2.00             +0.125%
        -------------------------------------------------------------------------------------------------

        Level 3          (greater than or equal to) 2.00 but (less than) 2.50             +0.125%
        -------------------------------------------------------------------------------------------------

        Level 4          (greater than or equal to) 2.50 but (less than) 3.00             +0.125%
        -------------------------------------------------------------------------------------------------
</TABLE>


<PAGE>   4

<TABLE>
<CAPTION>

        -------------------------------------------------------------------------------------------------
        Level            Leverage Ratio                                           Applicable Utilization
        -----            --------------                                                  Fee Rate
                                                                                         --------
        -------------------------------------------------------------------------------------------------
        <S>              <C>                                                      <C>
        Level 5          (greater than or equal to) 3.00                                  +0.125%
        -------------------------------------------------------------------------------------------------
</TABLE>

                  The Applicable Utilization Fee Rate shall be effective from
         and including the date on which the Agent receives a Compliance
         Certificate to but excluding the date on which the Agent receives the
         next Compliance Certificate; PROVIDED, HOWEVER, that if the Agent does
         not receive a Compliance Certificate by the date required by SECTION
         5.1, the Applicable Utilization Fee Rate shall, effective as of such
         date, be Level 5 to but excluding the date the Agent receives such
         Compliance Certificate.

                  (d) The first parenthetical phrase in the definition of the
term "Interest Period" in Section 1.1 of the Credit Agreement is hereby amended
by deleting the words "or twelve" therefrom.

                  (e) The definition of the term "Revolving Termination Date" in
Section 1.1 of the Credit Agreement is hereby amended by deleting the date
"September 24, 1999" from clause (a) thereof and replacing it with the date
"September 22, 2000."

                  2.3 AMENDMENT TO SECTION 2.9. Clause (c) of Section 2.9 of the
Credit Agreement is hereby amended by deleting ".05 of 1% (one percent) of" and
replacing it with "the Applicable Utilization Fee Rate times".

                  2.4 AMENDMENTS TO SECTION 5.5. Section 5.5 of the Credit
Agreement is hereby amended and restated to read in its entirety as follows:

                  "5.5 FINANCIAL INFORMATION

                           (a) The audited consolidated balance sheet as of
                  December 31, 1998 and the related consolidated statements of
                  income, of shareholders equity and of cash flow for the fiscal
                  year then ended, of the Company and its Subsidiaries, audited
                  by Ernst & Young LLP have been prepared in accordance with
                  GAAP consistently applied (except as disclosed therein)
                  throughout the period involved, present fairly the financial
                  position of the Company and such Subsidiaries as of the date
                  applicable and the results of their operations and cash flows
                  for the period then ended and show all material indebtedness
                  and other liabilities,

<PAGE>   5

                  direct or contingent, of the Company and its consolidated
                  Subsidiaries as of the date thereof, including liabilities for
                  taxes, material commitments and Contingent Obligations.

                           (b) The unaudited consolidated balance sheet as of
                  June 30, 1999 and the related consolidated statements of
                  income, of shareholders equity and of cash flow for the fiscal
                  quarter then ended of the Company and its Subsidiaries have
                  been prepared in accordance with GAAP consistently applied
                  (except as disclosed therein) throughout the period involved
                  and present fairly the financial position of the Company and
                  its Subsidiaries as of the date applicable and results their
                  operations and cash flows for the period then ended."

                  2.5 AMENDMENT TO SECTION 5.6. Section 5.6 of the Credit
Agreement is hereby amended by deleting the date "December 31, 1997" and
replacing it with "December 31, 1998."

                  2.6 AMENDMENT TO SCHEDULE 2.1. Schedule 2.1 to the Credit
Agreement is hereby amended and restated to read as set forth on Schedule 2.1
hereof.

         3. REPRESENTATIONS AND WARRANTIES. The Borrowers represent and warrant
to the Agent and the Banks that, on and as of the date hereof, and after giving
effect to this First Amendment:

                  3.1 AUTHORIZATION. The execution, delivery and performance by
the Borrowers of this First Amendment have been duly authorized by all necessary
corporate action, and this First Amendment has been duly executed and delivered
by the Borrowers.

                  3.2 BINDING OBLIGATION. This First Amendment constitutes the
legal, valid and binding obligation of the Borrowers, enforceable against the
Borrowers in accordance with its terms, except as enforceability may be limited
by applicable bankruptcy, insolvency, or similar laws affecting the enforcement
of creditors' rights generally or by equitable principles relating to
enforceability.

                  3.3 NO LEGAL OBSTACLE TO AMENDMENT. The execution, delivery
and performance of this First Amendment will not (a) contravene the Organization
Documents of either Borrower; (b) constitute a breach or default under any
material Contractual Obligation or violate or contravene any law or governmental
regulation or court decree or order binding on or affecting either Borrower
which individually or in the aggregate could reasonably be expected to have a
Material Adverse Effect; or


                                       5
<PAGE>   6


(c) result in, or require the creation or imposition of, any Lien on any of
either Borrower's properties. No approval or authorization of any governmental
authority is required to permit the execution, delivery or performance by the
Borrowers of this First Amendment, or the transactions contemplated hereby.

                  3.4 INCORPORATION OF CERTAIN REPRESENTATIONS. After giving
effect to the terms of this First Amendment, the representations and warranties
of the Company set forth in Article V of the Credit Agreement are true and
correct in all respects on and as of the date hereof as though made on and as of
the date hereof, except as to such representations made as of an earlier
specified date.

                  3.5 DEFAULT. No Default or Event of Default under the Credit
Agreement has occurred and is continuing.

         4. CONDITIONS, EFFECTIVENESS. The effectiveness of this First Amendment
shall be subject to the compliance by the Borrowers with their agreements herein
contained, and to the delivery of the following to Agent in form and substance
satisfactory to Agent:

                  4.1 AUTHORIZED SIGNATORIES. A certificate, signed by the
Secretary or an Assistant Secretary of each of the Borrowers and dated the date
of this First Amendment, as to the incumbency of the person or persons
authorized to execute and deliver this First Amendment and any instrument or
agreement required hereunder on behalf of the Borrowers.

                  4.2 NOTES. A Note in favor of each Bank duly executed by the
Borrowers.

                  4.3 GUARANTOR AFFIRMATION. An acknowledgment and reaffirmation
letter in the form of EXHIBIT B hereto duly executed by each party to the
Guaranty (a "Guarantor").

                  4.4 LEGAL OPINIONS. An opinion of counsel to the Borrowers and
the Guarantor, addressed to the Agent and the Banks, in a form reasonably
satisfactory to the Agent.

                  4.5 OTHER EVIDENCE. Such other evidence with respect to the
Borrowers or any other person as the Agent or any Bank may reasonably request to
establish the consummation of the trans-actions contemplated hereby, the taking
of all corporate action in connection with this First Amendment and the Credit
Agreement and the compliance with the conditions set forth herein.


                                       6
<PAGE>   7

         5.       MISCELLANEOUS.

                  5.1 EFFECTIVENESS OF THE CREDIT AGREEMENT AND THE NOTES.
Except as hereby expressly amended, the Credit Agreement shall each remain in
full force and effect and is hereby ratified and confirmed in all respects on
and as of the date hereof.

                  5.2 WAIVERS. This First Amendment is limited solely to the
matters expressly set forth herein and is specific in time and in intent and
does not constitute, nor should it be construed as, a waiver or amendment of any
other term or condition, right, power or privilege under the Credit Agreement or
under any agreement, contract, indenture, document or instrument mentioned
therein; nor does it preclude or prejudice any rights of the Agent or the Banks
thereunder, or any exercise thereof or the exercise of any other right, power or
privilege, nor shall it require the Majority Banks to agree to an amendment,
waiver or consent for a similar transaction or on a future occasion, nor shall
any future waiver of any right, power, privilege or default hereunder, or under
any agreement, contract, indenture, document or instrument mentioned in the
Credit Agreement, constitute a waiver of any other right, power, privilege or
default of the same or of any other term or provision.

                  5.3 COUNTERPARTS. This First Amendment may be executed in any
number of counterparts, and all of such counterparts taken together shall be
deemed to constitute one and the same instrument. This First Amendment shall not
become effective until the Borrowers, the Agent and the Majority Banks shall
have signed a copy hereof and the same shall have been delivered to the Agent.
Delivery of an executed counterpart of a signature page to this First Amendment
should be effective as delivery of a manually executed counterpart of this First
Amendment.

                  5.4 GOVERNING LAW. This First Amendment shall be governed by
and construed in accordance with the laws of the State of New York.

                  5.5 SEVERABILITY. The illegality or unenforceability of any
provision of this First Amendment or any instrument or agreement required
hereunder shall not in any way affect or impair the legality or enforceability
of the remaining provisions of this First Amendment or any instrument or
agreement required hereunder.


                                       7
<PAGE>   8

         IN WITNESS WHEREOF, the parties hereto have caused this First Amendment
to be duly executed and delivered by their proper and duly authorized officers
as of the day and year first above written.

                         HCR MANOR CARE, INC.



                         By: ___________________________
                         Title: ________________________


                         MANOR CARE, INC.


                         By: ___________________________
                         Title: ________________________


                         BANK OF AMERICA, NATIONAL
                    ASSOCIATION, as Agent


                         By: ___________________________
                         Title: ________________________


                         BANK OF AMERICA, NATIONAL
                    ASSOCIATION, as a Bank


                         By: ___________________________
                         Title: __________________________


                         THE CHASE MANHATTAN BANK


                         By: ___________________________
                         Title: ________________________


                                       8
<PAGE>   9

                         DEUTSCHE BANK AG, NEW YORK BRANCH
                         AND/OR CAYMAN ISLANDS BRANCH


                         By: ___________________________
                         Title: ________________________


                         By: ___________________________
                         Title: ________________________


                         FLEET NATIONAL BANK


                         By: ___________________________
                         Title: ________________________


                         THE HUNTINGTON NATIONAL BANK


                         By: ___________________________
                         Title: ________________________


                         ALLFIRST BANK


                         By: ___________________________
                         Title: ________________________


                         BANK OF MONTREAL


                         By: ___________________________
                         Title: ________________________


                         THE BANK OF NEW YORK


                         By: ___________________________
                         Title: ________________________


                                       9
<PAGE>   10

                         NATIONAL CITY BANK


                         By: ___________________________
                         Title: ________________________



                         WACHOVIA BANK, N.A.


                         By: ___________________________
                         Title: ________________________


                         THE FIFTH THIRD BANK


                         By: ___________________________
                         Title: ________________________


                         BANK ONE, N.A.


                         By: ___________________________
                         Title: ________________________


                         SUNTRUST BANK, CENTRAL FLORIDA,
                         NATIONAL ASSOCIATION


                         By: ___________________________
                         Title: ________________________


                                       10
<PAGE>   11

                                  SCHEDULE 2.1

                         COMMITMENTS AND PRO RATA SHARES
                         -------------------------------
<TABLE>
<CAPTION>


BANK                                                          COMMITMENT        SHARE
----                                                          ----------        -----
<S>                                                          <C>               <C>
Bank of America, National Association                        $ 35,000,000      17.5000%

The Chase Manhattan Bank                                     $ 25,000,000      12.5000%

Deutsche Bank AG                                             $ 25,000,000      12.5000%

Fleet National Bank                                          $ 18,000,000       9.0000%

The Huntington National Bank                                 $ 16,375,000       8.1875%

AllFirst Bank                                                $ 11,250,000       5.6250%

Bank of Montreal                                             $ 11,250,000       5.6250%

The Bank of New York                                         $ 11,250,000       5.6250%

National City Bank                                           $ 11,250,000       5.6250%

Wachovia Bank, N.A.                                          $ 11,250,000       5.6250%

The Fifth Third Bank                                         $ 9,375,000        4.6875%

Bank One, N.A.                                               $ 7,500,000        3.7500%

SunTrust Bank, Central Florida, National Association         $ 7,500,000        3.7500%

TOTAL                                                        $200,000,000         100%

</TABLE>

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