<PAGE>

                        THE PREMCOR REFINING GROUP INC.


                                AMENDMENT NO. 4
                              TO CREDIT AGREEMENT


          This AMENDMENT NO. 4 (the "Amendment") is dated as of May 11, 2001
                                     ---------
and entered into by and among The Premcor Refining Group Inc. (f/k/a Clark
Refining & Marketing, Inc.), a Delaware corporation (the "Company"), Bankers
                                                          -------
Trust Company, a New York Banking corporation, as Administrative Agent and
Collateral Agent, The Toronto-Dominion Bank, a Canadian chartered bank, as
Syndication Agent, and Fleet National Bank (f/k/a BankBoston, N.A.), a national
banking association ("Fleet"), as Documentation Agent, and the other financial
                      -----
institutions party hereto (the "Banks").  This Amendment amends the Credit
                                -----
Agreement (as amended, amended and restated, supplemented or otherwise modified,
the "Credit Agreement") dated as of November 19, 1999 by and among the parties
     ----------------
hereto.  Capitalized terms used herein without definition shall have the same
meanings herein as set forth in the Credit Agreement.

                                   RECITALS

          WHEREAS, the parties hereto entered into the Credit Agreement, which
provides for a loan facility to the Company; and

          WHEREAS, the Company has requested and the Banks have agreed to extend
the Facility Expiry Date under the Credit Agreement on the terms and subject to
the provisions set forth below.

          NOW, THEREFORE, in consideration of the premises and the agreements,
provisions and covenants herein contained, the parties hereto agree as follows:

                                   ARTICLE I

                      AMENDMENTS TO THE CREDIT AGREEMENT

     1.01 Amendment to Section 1.01 of the Credit Agreement.  The defined term
          -------------------------------------------------
"Facility Expiry Date" shall be amended by deleting the words "June 30, 2001"
 --------------------
therein and inserting the words "October 31, 2001" in lieu thereof.

     1.02 Amendment to Section 2.06(c) of the Credit Agreement.  Section 2.06(c)
          ----------------------------------------------------
of the Credit Agreement shall be amended by adding the following sentence at the
end thereof: "Notwithstanding anything herein to the contrary, in the event the
Company
<PAGE>

elects to reduce the Commitments in part in accordance with the terms hereof,
the Company may elect to reduce the Commitment of any Bank without reducing the
Commitments of the other Banks, provided, however, that the aggregate amount of
Commitments being reduced pursuant to this sentence shall not exceed
$70,000,000."


                                  ARTICLE II

                          EFFECTIVENESS OF AMENDMENTS

          This Amendment shall become effective on the opening of business in
New York on the Business Day (the "Effective Date") on which the Administrative
                                   --------------
Agent has notified the Company and the Banks that the Administrative Agent has
executed a counterpart signature page of this Amendment and has received
executed counterpart signature pages of this Amendment from the Company and each
of the Banks.

                                  ARTICLE III

                                 MISCELLANEOUS

     3.01 Fees and Expenses.  The Company agrees to pay the following:
          -----------------

     (a)  An amendment fee to each Bank that consents to this Amendment and
          returns an executed signature page evidencing the foregoing, equal to
          0.25% of such Bank's Commitment, which fee shall be due and payable
          upon the effectiveness of this Amendment; and

     (b)  All reasonable expenses incurred by the Administrative Agent in
          connection with the preparation, execution and delivery of this
          Amendment, including, without limitation, reasonable fees and expenses
          of counsel to the Administrative Agent.

     3.02 Reference to and Effect on the Credit Agreement and the Other Loan
          ------------------------------------------------------------------
Documents.
---------

     (a)  This Amendment modifies the Credit Agreement to the extent set forth
          herein, is hereby incorporated by reference into the Credit Agreement
          and is made a part thereof. On and after the effective date, each
          reference in the Credit Agreement to "this Agreement", "hereunder",
          "hereof", "herein" or words of like import referring to the Credit
          Agreement, and each reference in the other Loan Documents to the
          "Credit Agreement", "thereunder", "thereof" or words of like import
          referring to the Credit
<PAGE>

          Agreement shall mean and be a reference to the Credit Agreement as
          amended by this Amendment.

     (b)  Except as specifically amended by this Amendment, the Credit Agreement
          and the other Loan Documents shall remain in full force and effect and
          are hereby ratified and confirmed.

     (c)  The execution, delivery and performance of this Amendment shall not,
          except as expressly provided herein, constitute a waiver or amendment
          of any provision of, or operate as a waiver of any right, power or
          remedy of the Administrative Agent, any Bank or any Issuing Bank
          under, the Credit Agreement or any of the other Loan Documents.

     3.02 Representations and Warranties; No Default or Event of Default.  On
          --------------------------------------------------------------
the date of effectiveness of any of the amendments and waivers herein (after
giving effect to the consummation of the transactions contemplated by this
Amendment to have occurred on or prior to such date), the Company shall be
deemed to have certified to the Banks that, after giving effect to the
amendments and waivers contained herein that become effective on such date (i)
all of the representations and warranties contained in the Credit Agreement are
true and correct on and as of the date thereof with the same effect as if made
on and as of such date (except to the extent such representations and warranties
expressly refer to an earlier date, in which case they shall be true and correct
as of such earlier date and except to the extent (x) the representations and
warranties set forth in Section 6.05 of the Credit Agreement relate to any
litigation which has been specifically disclosed to the Banks and which has been
added to Schedule 6.05 to the Credit Agreement with the written approval of the
         -------------
Majority Banks and (y) the representation and warranty set forth in Section 6.25
of the Credit Agreement relates to any event or condition which has been
specifically disclosed to the Banks and which has been added to Schedule 6.25 to
                                                                -------------
the Credit Agreement with the written approval of the Majority Banks) and (ii)
no Default or Event of Default exists as of the Effective Date.

     3.03 Headings.   Section and subsection headings in this Amendment are
          --------
included herein for convenience of reference only and shall not constitute a
part of this Amendment for any other purpose or be given any substantive effect.

     3.04 Applicable Law.  THIS AMENDMENT SHALL BE GOVERNED BY, AND SHALL BE
          --------------
CONSTRUED AND ENFORCED IN ACCORDANCE WITH, THE INTERNAL LAWS OF THE STATE OF NEW
YORK, WITHOUT REGARD TO CONFLICT OF LAWS PRINCIPLES.

     3.05 Counterparts.  This Amendment may be executed in any number of
          ------------
counterparts and by different parties hereto in separate counterparts, each of
which when so executed and delivered shall be deemed an original, but all such
counterparts together shall constitute one and the same instrument.
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                 [REMAINDER OF PAGE INTENTIONALLY LEFT BLANK]
<PAGE>

          IN WITNESS WHEREOF, the parties hereto have caused this Amendment to
be duly executed and delivered by their respective officers thereunto duly
authorized as of the date first written above.


                    THE PREMCOR REFINING GROUP INC.


                    By:_______________________________
                       Name:
                       Title:



                                    AGENTS
                                    ------


                    BANKERS TRUST COMPANY
                      as Administrative Agent
                      and Collateral Agent


                    By:_______________________________
                       Name:
                       Title:




                    THE TORONTO DOMINION BANK
                      as Syndication Agent


                    By:_______________________________
                       Name:
                       Title:
<PAGE>

                    FLEET NATIONAL BANK
                     (f/k/a BankBoston, N.A.)
                      as Documentation Agent


                    By:_______________________________
                       Name:
                       Title:


                                     BANKS
                                     -----


                    ABN AMRO BANK
                     as a Bank


                    By:_______________________________
                       Name:
                       Title:


                    By:_______________________________
                       Name:
                       Title:



                    ARAB BANKING CORPORATION (B.S.C.)
                     as a Bank


                    By:_______________________________
                       Name:
                       Title:
<PAGE>

                    BANKERS TRUST COMPANY
                     as a Bank


                    By:_______________________________
                       Name:
                       Title:




                    COMERICA BANK
                     as a Bank


                    By:_______________________________
                       Name:
                       Title:



                    CONGRESS FINANCIAL CORPORATION
                     as a Bank


                    By:_______________________________
                       Name:
                       Title:



                    CREDIT LYONNAIS NEW YORK BRANCH
                     as a Bank


                    By:_______________________________
                       Name:
                       Title:



                    FLEET NATIONAL BANK
<PAGE>

                     (f/k/a BankBoston, N.A.)
                      as a Bank


                    By:_______________________________
                       Name:
                       Title:


                    THE FUJI BANK, LIMITED
                     as a Bank


                    By:_______________________________
                       Name:
                       Title:



                    GMAC COMMERCIAL CREDIT LLC
                     as a Bank


                    By:_______________________________
                       Name:
                       Title:
<PAGE>

                    HELLER FINANCIAL, INC.
                     as a Bank


                    By:_______________________________
                       Name:
                       Title:


                    HIBERNIA NATIONAL BANK
                     as a Bank


                    By:_______________________________
                       Name:
                       Title:



                    JACKSON NATIONAL LIFE INSURANCE COMPANY
                     as a Bank


                    By: PPM Finance, Inc.,
                        as Attorney-in-Fact

                    By:_______________________________
                       Name:
                       Title:



                    SIEMENS CREDIT CORP.
                     as a Bank


                    By:_______________________________
                       Name:
                       Title:
<PAGE>

                    SOCIETE GENERALE, SOUTHWEST AGENCY
                     as a Bank


                    By:_______________________________
                       Name:
                       Title:


                    SOVEREIGN BANK
                     as a Bank


                    By:_______________________________
                       Name:
                       Title:


                    TEXTRON FINANCIAL CORPORATION
                      as a Bank


                    By:_______________________________
                       Name:
                       Title:


                    TORONTO DOMINION (TEXAS), INC.
                      as a Bank


                    By:_______________________________
                       Name:
                       Title:


                    TRANSAMERICA BUSINESS CREDIT CORPORATION
                     as a Bank


                    By:_______________________________
                       Name:
                       Title:
<PAGE>

                    UNION BANK OF CALIFORNIA, N.A.
                     as a Bank


                    By:_______________________________
                       Name:
                       Title:


                    WELLS FARGO BANK (TEXAS), N.A.
                     as a Bank


                    By:_______________________________
                       Name:
                       Title:
<PAGE>

                                 ISSUING BANKS
                                 -------------

                    BANKERS TRUST COMPANY
                      as Issuing Bank


                    By:_______________________________
                       Name:
                       Title:



                    THE TORONTO DOMINION BANK
                      as Issuing Bank


                    By:_______________________________
                       Name:
                       Title:


                    FLEET NATIONAL BANK
                     (f/k/a BankBoston, N.A.)
                      as Issuing Bank

                    By:_______________________________
                       Name:
                       Title:


                    ABN AMRO BANK
                     as Issuing Bank


                    By:_______________________________
                       Name:
                       Title:
<PAGE>

                    COMERICA BANK DETROIT
                     as Issuing Bank

                    By:_______________________________
                       Name:
                       Title: