<PAGE>
 
                                                                  EXHIBIT 10.54
 
                               AMENDMENT NO. 4
                                      TO
                      VASTAR RESOURCES, INC. SAVINGS PLAN
                          __________________________


Pursuant to the power of amendment reserved therein, the Vastar Resources, Inc.
Savings Plan (the "Plan") is hereby amended effective as of July 1, 1997:


Paragraph 8.1 of the Plan is amended to read as follows:

"8.1 Termination of Employment
     (a)  If a Member's membership in the Plan is terminated due to disability,
          termination of employment for any other reason except death, or as the
          result of a sale described in Subparagraphs 2.3(b) or (c), the Member
          shall receive all items in the Member's Account.  Each Member shall be
          fully vested at all times in all items in the Member's Account,
          whether the same be derived from Member's Contributions, Company
          contributions, or earnings thereon.

     (b)  Upon the election of the Member who has terminated employment, all
          items in a Member's Account shall be distributed to the Member.  With
          respect to a Member who does not request a distribution:

          (i)   Notwithstanding anything to the contrary in this Paragraph 8.1
                and subject to the provisions of Paragraph 8.7, a Member's
                Account shall be distributed the later of age 65, or 12 months
                following termination of membership under Subparagraph 8.1(a);



          (ii)  In the case of the Member's death prior to final distribution,
                the Member's Account shall be distributed in accordance with
                Paragraph 8.2 of the Plan; and


                                       1
<PAGE>
 
          (iii) No loans or partial withdrawals may be taken following
                termination of employment or disability.

     (c)  Notwithstanding anything to the contrary in this Paragraph 8.1, all
          items in the Account of a Member who has terminated employment, and
          whose Account balance is $3,500 or less, shall be distributed 12
          months following the Member's termination of membership, unless the
          Member elects an earlier distribution date.

     (d)  Notwithstanding anything in the Plan to the contrary, when a Member
          elects to receive all items in the Member's Account and, in
          conjunction therewith, directs that items in his or her Account be
          converted pursuant to Paragraph 6.4 or 6.5, the conversion shall be
          transacted on the first transaction date under the Plan following the
          Administrator's receipt of a request for distribution."


Executed this 30th day of December, 1997.


ATTEST                                        VASTAR RESOURCES, INC.



BY: /s/ JONATHAN D. EDELFELT                      By: /s/ JEFFREY M. BENDER
    -------------------------                         ---------------------
    JONATHAN D. EDELFELT                              JEFFREY M. BENDER
    Associate Secretary                               Vice President
                                                      Human Resources

                                       2