<PAGE>

                                                                   EXHIBIT 10(B)

                               FOURTH AMENDMENT
                               ----------------


     THIS FOURTH AMENDMENT dated as of May 12, 2000 (this "Amendment") amends
                                                           ---------
the Credit Agreement dated as of September  29, 1998 (as previously amended, the
"Credit Agreement") among United Rentals (North America), Inc. (the "Company"),
 ----------------                                                    -------
United Rentals, Inc. ("Parent"), United Rentals of Canada, Inc., various
                       ------
financial institutions, Bank of America Canada, as Canadian Agent, and Bank of
America, N.A. (formerly known as Bank of America National Trust and Savings
Association), as U.S. Agent.  Terms defined in the Credit Agreement are, unless
otherwise defined herein or the context otherwise requires, used herein as
defined therein.

     WHEREAS, the parties hereto desire to amend the Credit Agreement as set
forth herein;

     NOW, THEREFORE, the parties hereto agree as follows:

1    SECTION   Amendments.  Effective on (and subject to the occurrence of)
               ----------
the  Amendment Effective Date (as defined below), the Credit Agreement shall be
amended as set forth in this Section 1.
                             ---------

     1.1  Amendment to Cover Page.  The cover page is amended by deleting the
          -----------------------
language "BANKBOSTON, N.A., as Co-Agent," therefrom and substituting the
language "FLEET NATIONAL BANK," therefor (so that Fleet National Bank is
designated a Documentation Agent).

     1.2  Amendments to Definition of Debt.  The definition of Debt in Section
          --------------------------------
1.1 is amended by (a) deleting the word "and" at the end of clause (f) and
substituting a comma therefor; and (b) adding the following language after the
word "Person" at the end of clause (g): "and (h) all Synthetic Lease Obligations
of such Person".

     1.3  Amendment of Definition of Funded Debt.  The definition of Funded Debt
          --------------------------------------
in Section 1.1 is amended in its entirety to read as follows:

          "Funded Debt means (a) all Debt of Parent and its Subsidiaries and
           -----------
     (b) to the extent not included in the definition of Debt, all outstanding
     Securitization Obligations, but excluding (i) contingent obligations in
     respect of undrawn letters of credit and Suretyship Liabilities (except to
     the extent constituting contingent obligations or Suretyship Liabilities in
     respect of Funded Debt of a Person other than Parent or any Subsidiary),
     (ii) Hedging Obligations, (iii) Debt of Parent to Subsidiaries and Debt of
     Subsidiaries to Parent or to other Subsidiaries and (iv) Debt (including
     guaranties thereof) in respect of the QuIPS Debentures and the QuIPS
     Preferred Securities."
<PAGE>

     1.4  Amendment to Definition of Interest Coverage Ratio.  Clause (b) of the
          --------------------------------------------------
definition of Interest Coverage Ratio in Section 1.1 is amended by inserting the
language "(without duplication)" after the word "plus" therein.
                                                 ----

     1.5  Amendment to Definition of Interest Expense.  The definition of
          -------------------------------------------
Interest Expense in Section 1.1 is amended by adding the following after the
words "imputed interest on Capital Leases" in clause (a) thereof: "and Synthetic
Leases".

     1.6  Amendments to Definition of Interest Period.  The definition of
          -------------------------------------------
Interest Period in Section 1.1 is amended by (a) adding the following language
after the words "six months thereafter" in clause (a) thereof:  "(or, in the
case of the Company, two weeks thereafter)"; and (b) adding the following
language after the words "for a Eurodollar Loan" in the first line of clause
(ii) of the proviso thereto: "(other than a Eurodollar Loan with a two-week
Interest Period)".

     1.7  Amendment of Definition of Term Loan Agreement.  The definition of the
          ----------------------------------------------
Term Loan Agreement in Section 1.1 is amended in its entirety to read as
follows:

          "Term Loan Agreement" means the Amended and Restated Term Loan
           -------------------
     Agreement dated as of May 12, 2000 among Parent, the Company, various
     financial institutions and BofA, as Agent, which amends and restates the
     Term Loan Agreement dated as of July 10, 1998."

     1.8  Amendment of Definition of Permitted Senior Secured Debt.  The
          --------------------------------------------------------
definition of Permitted Senior Secured Debt in Section 1.1 is amended in its
entirety to read as follows:

          "Permitted Senior Secured Debt means any Debt arising under (a) the
           -----------------------------
     Amended and Restated Term Loan Agreement dated as of May 12, 2000 among
     Parent, the Company, various financial institutions and BofA, as Agent
     which amends and restates the Term Loan Agreement dated as of July 15,
     1999; and (b) any other term loan agreement (other than the Term Loan
     Agreement as defined herein) among Parent, the Company, various financial
     institutions and BofA, as agent; provided that (i) any such other term loan
                                      --------
     agreement shall contain covenants and defaults which are no more
     restrictive for Parent and its Subsidiaries than the covenants and defaults
     contained in this Agreement, (ii) any such Debt shall be issued on or prior
     to December 31, 2000, shall mature no earlier than June 30, 2006 and shall
     have amortization of no more than 20% of the principal amount thereof prior
     to July 15, 2005, (iii) any such Debt shall constitute "Senior
     Indebtedness" as defined in each Subordinated Note Indenture and (iv) no
     Debt under any such other term loan agreement shall have interest rate
     spreads greater than the then-applicable interest rate spreads under the
     Term Loan Agreement referred to in clause (a) above."
                                        ----------
<PAGE>

     1.9  Amendment to Definition of Reference Rate.  The definition of
          -----------------------------------------
"Reference Rate" in Section 1.1 is amended by deleting the word "reference"
therein and substituting the word "prime" therefor.

     1.10 Amendment to Definition of Securitization Transaction.  The
          -----------------------------------------------------
definition of "Securitization Transaction" in Section 1.1 is amended in its
entirety to read as follows:

          "Securitization Transaction means an Equipment Securitization
           --------------------------
     Transaction or a Receivables Securitization Transaction."

     1.11 Amendment to Definition of Special Purpose Vehicle.  The definition of
          --------------------------------------------------
"Special Purpose Vehicle" in Section 1.1 is amended in its entirety to read as
follows:

          "Special Purpose Vehicle means an ES Special Purpose Vehicle or an RS
           -----------------------
     Special Purpose Vehicle."

     1.12 Addition of Definitions.  Section 1.1 is amended by adding the
          -----------------------
following definitions in appropriate alphabetical sequence:

          "Equipment Securitization Transaction means any sale, assignment,
           ------------------------------------
     pledge or other transfer (a) by Parent or any Subsidiary of equipment or
     related assets, (b) by any ES Special Purpose Subsidiary of leases or
     rental agreements between Parent and/or any Subsidiary, as lessee, on the
     one hand, and such ES Special Purpose Vehicle, as lessor, on the other
     hand, relating to such equipment or related assets, and lease receivables
     arising under such leases and rental agreements and (c) by Parent or any
     Subsidiary of any interest in any of the foregoing, together in each case
     with (i) any and all proceeds thereof (including all collections relating
     thereto, all payments and other rights under insurance policies or
     warranties relating thereto, all disposition proceeds received upon a sale
     thereof, and all rights under manufacturers' repurchase programs or
     guaranteed depreciation programs relating thereto), (ii) any collection or
     deposit account relating thereto, and (iii) any collateral, guaranties,
     credit enhancement or other property or claims supporting or securing
     payment on, or otherwise relating to, any such leases, rental agreements or
     lease receivables."

          "ES Special Purpose Vehicle means a trust, bankruptcy remote entity or
           --------------------------
     other special purpose entity which is a Subsidiary of Parent (or, if not a
     Subsidiary, the common equity of which is wholly-owned, directly or
     indirectly, by Parent) and which is formed for the purpose of, and engages
     in no material business other than, acting as a lessor, issuer or depositor
     in an Equipment Securitization Transaction (and, in connection therewith,
     owning the equipment, leases, rental agreements, lease receivables, rights
     to payment and other interests, rights and assets described in the
     definition of Equipment Securitization Transaction, and pledging or
     transferring any of the foregoing or interests therein)."
<PAGE>

          "Floor Plan Financing Arrangement means any arrangement whereby
           --------------------------------
     Parent or a Subsidiary grants a Lien to an equipment manufacturer (or an
     affiliate thereof which is in the financing business) on all equipment
     purchased from such manufacturer and the proceeds thereof, including
     equipment which was not financed by such manufacturer (or an affiliate
     thereof)."

          "Receivables Securitization Transaction means any sale, assignment
           --------------------------------------
     or other transfer by Parent or any Subsidiary of accounts receivable, lease
     receivables or other payment obligations owing to Parent or such Subsidiary
     or any interest in any of the foregoing, together in each case with any
     collections and other proceeds thereof, any collection or deposit account
     related thereto, and any collateral, guaranties or other property or claims
     supporting or securing payment by the obligor thereon of, or otherwise
     related to, or subject to leases giving rise to, any such receivables."

          "RS Special Purpose Vehicle means a trust, bankruptcy remote entity or
           --------------------------
     other special purpose entity which is a Subsidiary of Parent (or, if not a
     Subsidiary, the common equity of which is wholly-owned, directly or
     indirectly, by Parent) and which is formed for the purpose of, and engages
     in no material business other than, acting as an issuer or a depositor in a
     Receivables Securitization Transaction (and, in connection therewith,
     owning accounts receivable, lease receivables, other rights to payment,
     leases and related assets and pledging or transferring any of the foregoing
     or interests therein)."

          "Securitization Obligations means, with respect to any Securitization
           --------------------------
     Transaction, the aggregate investment or claim held at any time by all
     purchasers, assignees or transferees of (or of interests in) or holders of
     obligations that are supported or secured by (i) in the case of an
     Equipment Securitization Transaction, equipment or related assets (and
     leases, rental agreements, lease receivables, rights to payment and other
     interests, rights and assets described in the definition of Equipment
     Securitization Transaction) in connection with such Equipment
     Securitization Transaction, and (ii) in the case of a Receivables
     Securitization Transaction, accounts receivable, lease receivables and
     other rights to payment in connection with such Receivables Securitization
     Transaction."

          "Synthetic Lease means a lease transaction under which the parties
           ---------------
     intend that (i) the lease will be treated as an "operating lease" by the
     lessee pursuant to Statement of Financial Accounting Standards No. 13, as
     amended, and (ii) the lessee will be entitled to various tax and other
     benefits ordinarily available to owners (as opposed to lessees) of like
     property."

          "Synthetic Lease Obligations means, with respect to any Person, the
           ---------------------------
     sum of (a) all remaining rental obligations of such Person as lessee under
     Synthetic Leases which are attributable to principal and, without
     duplication, (b) all rental and purchase price payment obligations of such
     Person under Synthetic Leases assuming such Person exercises the option to
     purchase the leased property at the end of the lease term."
<PAGE>

     1.13 Amendment to Section 2.2.1.  Section 2.2.1 is amended by adding the
          --------------------------
following language after the words "to the Company" in clause (i) of the proviso
thereto: ", or two different Groups of Eurodollar Loans with two-week Interest
Periods,".

     1.14 Amendments to Section 6.1.2.  Section 6.1.2 is amended by (a) deleting
          ---------------------------
the date "December 31, 1998" therein and substituting the date "December 31,
2000" therefor; and (b) deleting the amount of "$850,000,000" therein and
substituting the amount "U.S.$922,500,000" therefor.

     1.15 Amendment to Section 10.6.1.  Section 10.6.1 is amended by deleting
          ---------------------------
the ratio "0.65 to 1" therein and substituting the ratio "0.675 to 1" therefor.

     1.16 Amendment to Section 10.6.2.  The table in Section 10.6.2 is amended
          ---------------------------
in its entirety to read as follows:

<TABLE>
<CAPTION>
           "Computation                        Interest
          Period Ending                  Coverage Ratio
          ---------------------------------------------
          <S>                            <C>
          3/31/99 through 6/30/01           1.75 to 1
          9/30/01 and thereafter            2.00 to 1."
</TABLE>


     1.17 Amendment to Section 10.6.4.  Section 10.6.4 is amended in its
          ---------------------------
entirety to read as follows:

          "10.6.4 Senior Debt to Tangible Assets.  Not permit the ratio of (i)
                  ------------------------------
     Senior Debt minus all Securitization Obligations to (ii) Tangible Assets
     minus, to the extent included in Tangible Assets, all assets which are
     owned by or subject to a Lien in favor of a Special Purpose Vehicle to
     exceed 1.0 to 1.0 at any time."

     1.18 Amendment to Section 10.6.5.  Section 10.6.5 is amended by deleting
          ---------------------------
the ratio "3.0 to 1.0" therein and substituting the ratio "2.75 to 1.0"
therefor.

     1.19 Amendments to Section 10.7.  Section 10.7 is amended by (a) deleting
          --------------------------
the amount "U.S.$750,000,000" in clause (j) and substituting the amount
"U.S.$1,000,000,000" therefor; (b) adding a comma followed by the following
language before the semi-colon at the end of clause (c) therein: "and provided,
                                                                      --------
further, that the aggregate amount of all such Debt arising in connection with
-------
Floor Plan Financing Arrangements shall not exceed U.S.$30,000,000"; and (c)
deleting the existing clauses (b), (e), (k) and (n) and substituting the
following therefor, respectively:

          "(b) unsecured Debt of Parent, the Company and Subsidiaries of the
     Company (excluding Contingent Payments and Seller Subordinated Debt);
     provided that no Subsidiary of the Company shall incur any such Debt if,
     --------
     after giving effect thereto, the
<PAGE>

     aggregate amount of all then-outstanding Debt of Subsidiaries of the
     Company permitted solely by this clause (b) would exceed 10% of Net Worth;"
                                      ----------

          "(e) unsecured Debt of any Special Purpose Vehicle to any Subsidiary
     of the Company;"

          "(k) Synthetic Lease Obligations, provided that the aggregate amount
     of all Synthetic Lease Obligations plus (without duplication) the aggregate
     amount of all Securitization Obligations arising under Equipment
     Securitization Transactions shall not at any time exceed the greater of
     U.S. $500,000,000 or 15% of Tangible Assets;"

          "(n) Debt in connection with Securitization Transactions."

     1.20 Amendments to Section 10.8.  Section 10.8 is amended by (a) deleting
          --------------------------
the reference to "clauses (a) and (j) of Section 10.7" in clause (g) thereof and
                  -----------     ---    ------------
substituting "clauses (a), (j) and (k) of Section 10.7" therefor; and (b)
              -----------  ---     ---    ------------
deleting the existing clauses (d) and (h) and substituting the following
therefor, respectively:

          "(d) Liens securing Debt permitted by clause (c) of Section 10.7 (and
                                                ----------    ------------
     attaching only to the property (i) being leased (in the case of Capital
     Leases), (ii) purchased from the relevant manufacturer (in the case of
     Floor Plan Financing Arrangements) or (iii) the purchase price for which
     was or is being financed by such Debt (in the case of other Debt) and, in
     each case, the proceeds (including insurance proceeds) of any disposition
     or loss of such property);"

          "(h) Liens arising in connection with Securitization Transactions."

     1.21 Amendments to Section 10.10.  Section 10.10 is amended in its entirety
          ---------------------------
to read as follows:

          "10.10 Restricted Payments.  Not, and not permit any Subsidiary to,
                 -------------------
     (a) declare or pay any dividends on any of its capital stock (other than
     stock dividends), (b) purchase or redeem any such stock or any warrants,
     units, options or other rights in respect of such stock, (c) make any other
     distribution to shareholders, (d) prepay, purchase, defease or redeem any
     Subordinated Debt, (e) make any payment of principal of or interest on, or
     acquire, redeem or otherwise retire, or make any other distribution in
     respect of, any of the QuIPS Debentures or the QuIPS Preferred Securities
     or (f) set aside funds for any of the foregoing; provided that (i) any
                                                      --------
     Subsidiary of the Company may declare and pay dividends to the Company or
     to any other wholly-owned Subsidiary of the Company; (ii) the Company may
     declare and pay dividends to Parent; (iii) the QuIPS Trust may make a
     distribution of Parent's common stock pursuant to the terms of the QuIPS
     Preferred Securities or the QuIPS Debentures; (iv) so long as no Event of
     Default or Unmatured Event of Default exists or would result therefrom,
     Parent may make payments on the QuIPS Debentures and permit the QuIPS Trust
     to make corresponding distributions on
<PAGE>

     the QuIPS Preferred Securities in accordance with the terms of the QuIPS
     Indenture; (v) so long as (x) no Event of Default or Unmatured Event of
     Default exists or would result therefrom and (y) the aggregate amount of
     all purchases made by Parent since April 30, 2000 (excluding purchases
     permitted by clause (vi) below) does not exceed U.S.$200,000,000, Parent
                  -----------
     may repurchase its capital stock; and (vi) so long as (x) no Event of
     Default or Unmatured Event of Default exists or would result therefrom and
     (y) the aggregate amount of all purchases of stock, warrants or units made
     by Parent (or, prior to August 5, 1998, the Company) since October 1, 1997
     (excluding purchases permitted by clause (v) above) does not exceed
                                       ----------
     U.S.$12,000,000, Parent may purchase its common stock or warrants, or units
     issued in respect thereof, from time to time on terms consistent with those
     set forth under the heading "Certain Agreements Relating to the Outstanding
     Securities" in the Company's Private Placement Memorandum dated September
     12, 1997; provided, further, that any stock of Parent, or warrants or units
               --------  -------
     relating thereto, purchased by Parent pursuant to clause (v) or (vi) above
                                                       ----------    ----
     shall be immediately retired. Nothing in this Section 10.10 shall prohibit
                                                   -------------
     Parent from permitting the cashless exercise of any options or warrants for
     stock of Parent."

     1.22 Amendments to Section 10.11.  Section 10.11 is amended by (a)
          ---------------------------
inserting the words "or exchanges" after the words "including sales" in the
first parenthetical clause therein; (b) inserting the words "or an asset
exchange of like-kind property" in clause (c) immediately after "(including
pursuant to a merger"; and (c) deleting clauses (d) and (e) in their entirety
and substituting the following therefor, respectively:

          "(d) the sale, assignment or other transfer of accounts receivable,
     lease receivables or other rights to payment pursuant to any Receivables
     Securitization Transaction or of equipment and related assets (and leases,
     rental agreements, lease receivables, rights to payment and other
     interests, rights and assets described in the definition of Equipment
     Securitization Transaction) pursuant to any Equipment Securitization
     Transaction, provided that the aggregate amount of all equipment and
                  --------
     related assets (and leases, rental agreements, lease receivables, rights to
     payment and other interests, rights and assets described in the definition
     of Equipment Securitization Transaction) sold, assigned or otherwise
     transferred pursuant to all Equipment Securitization Transactions in any
     Fiscal Year (in addition to any such assets which are permitted to be sold
     in such Fiscal Year pursuant to clause (e)) shall not exceed $25,000,000;
                                     ----------
     and (e) sales and dispositions of assets (including the stock of
     Subsidiaries and asset exchanges of like-kind property), in addition to
     sales and other dispositions in the ordinary course of business or
     permitted by clause (d), so long as the net book value of (i) all assets
                  ----------
     disposed of in like-kind exchanges in any Fiscal Year does not exceed 5% of
     the net book value of the consolidated assets of Parent and its
     Subsidiaries as of the last day of the preceding Fiscal Year and (ii) all
     other assets sold or otherwise disposed of in any Fiscal Year does not
     exceed 5% of the net book value of the consolidated assets of Parent and
     its Subsidiaries as of the last day of the preceding Fiscal Year."
<PAGE>

     1.23 Amendment to Section 10.13.  Section 10.13 is amended by adding the
          --------------------------
following immediately after the words "for general corporate purposes" therein:
"(including repurchases of stock of Parent to the extent permitted by Section
                                                                      -------
10.10)".
-----

     1.24 Amendment to Section 10.14.  Section 10.14 is amended by adding the
          --------------------------
following words after the words "other than" at the beginning of the
parenthetical clause in clause (ii) therein: "real property and".

     1.25 Amendments to Section 10.21.  Section 10.21 is amended by (a) deleting
          ---------------------------
the amount "$5,000,000" in clause (k) and substituting the amount
"U.S.$10,000,000" therefor; and (b) deleting clauses (d) and (i) and
substituting the following therefor, respectively:

          "(d) in the ordinary course of business, Investments by the Company in
               Parent or in any Subsidiary of the Company or by any of the
               Subsidiaries of the Company in Parent, the Company or any other
               Subsidiary of the Company, by way of intercompany loans, advances
               or guaranties, all to the extent permitted by Section 10.7;"
                                                             ------------

          "(i) Investments by Parent in the QuIPS Trust existing on the date
               hereof;".

     1.26 Amendment of Section 10.23.  Section 10.23 is amended in its entirety
          --------------------------
to read as follows:

          "10.23 Quips Documents.  Not permit any amendment to or modification
                 ---------------
     of the QuIPS Debentures, the QuIPS Preferred Securities, either QuIPS
     Guarantee or the QuIPS Indenture which, in any such case, is adverse to the
     interests of the Banks."

     1.27 Addition of Section 10.24.  The following Section 10.24 is added in
          -------------------------
appropriate numerical sequence:

          "10.24 Limitations on Securitization Transactions.  Not at any time
                 ------------------------------------------
     permit the aggregate amount of all Securitization Obligations to exceed (a)
     in the case of Receivables Securitization Transactions, U.S.$150,000,000;
     and (b) in the case of Equipment Securitization Transactions, the remainder
     of (i) the greater of (x) U.S.$500,000,000 and (y) 15% of Tangible Assets
     minus (ii) the aggregate amount of all Synthetic Lease Obligations
     -----
     (excluding, to prevent double-counting, Synthetic Lease Obligations which
     also are Securitization Obligations arising under Equipment Securitization
     Transactions) of Parent and its Subsidiaries."


     1.28 Amendment to Section 12.1.5.  Section 12.1.5 is amended by deleting
          ---------------------------
the language "or 10.16" therein and substituting a comma therefor followed by
                 -----
the language "10.16 or 10.24".
              -----    -----
<PAGE>

     1.29 Amendment of Section 13.11.  Section 13.11 is amended in its entirety
          --------------------------
to read as follows:

          "13.11 Collateral Matters.  The Banks irrevocably authorize each
                 ------------------
     Agent, at its option and in its discretion, (a) to release any Lien granted
     to or held by such Agent under any Collateral Document (i) upon termination
     of the Commitments and payment in full of all Loans and all other
     obligations of the Borrowers hereunder and the expiration or termination of
     all Letters of Credit; (ii) covering property sold or to be sold or
     disposed of as part of or in connection with any disposition permitted
     hereunder; or (iii) subject to Section 15.1, if approved, authorized or
                                    ------------
     ratified in writing by the Required Banks; and (b) to subordinate any Lien
     on property granted to or held by such Agent under any Collateral Document
     to the holder of any Lien on such property which is permitted by clause (d)
                                                                      ----------
     or (h) of Section 10.8. Upon request by either Agent at any time, the Banks
        ---    ------------
     will confirm in writing such Agent's authority to release, or subordinate
     its Lien on, particular types or items of collateral pursuant to this
     Section 13.11."
     -------------

     1.30 Amendment of Schedule 1.1(C). Schedule 1.1(C) to the Credit Agreement
          ----------------------------
is amended in its entirety to read as set forth in Schedule 1.1(C) to this
                                                   ---------------
Amendment.

2    SECTION   Representations and Warranties. Parent and the Company represent
               ------------------------------
and warrant to the Agents and the Banks that (a) each of the representations and
warranties made by Parent, the Company and each Subsidiary in Section 9
(excluding Section 9.8) of the Credit Agreement and the other Loan Documents, as
amended hereby (as so amended, the "Amended Agreement"), is true and correct as
                                    -----------------
of the date hereof, with the same effect as if made on such date (except to the
extent stated to relate to an earlier date, in which case such representations
and warranties shall be true and correct in all material respects as of such
earlier date), and (b) no Event of Default or Unmatured Event of Default has
occurred or is continuing.

3    SECTION   Effectiveness.  The amendments set forth in Section 1 above shall
               -------------                               ---------
become effective on the date (the "Amendment Effective Date") when the U.S.
                                   ------------------------
Agent shall have received (a) counterparts hereof executed by the Company,
Parent, the Required Banks and the U.S. Agent (provided that Section 1.14 (the
                                               --------
amendments to Section 6.1.2) shall become effective only when the U.S. Agent
shall have received counterparts hereof executed by all U.S. Banks and all other
conditions precedent to the effectiveness hereof have been satisfied); (b)
confirmation that substantially similar amendments to each of the Term Loan
Agreement and the term loan agreement relating to the existing Permitted Senior
Secured Debt have (or concurrently with the effectiveness hereof will) become
effective; (c) a Confirmation in the form of Exhibit A hereto signed by Parent,
                                             ---------
the Company and all U.S. Subsidiaries (other than the QuIPS Trust); (d) the
opinions of Weil, Gotshal & Manges LLP and Oscar D. Folger, substantially in the
form of Exhibits B and C hereto, respectively; and (e) an amendment fee for each
        ----------     -
U.S. Bank that has executed and delivered (by facsimile or otherwise) to the
U.S. Agent a counterpart hereof prior to 2:00 p.m., Chicago time, on May 12,
2000, an amendment fee in an amount equal to 0.25% of such Bank's Commitment.

4    SECTION   Miscellaneous.
               -------------
<PAGE>

     4.1  Continuing Effectiveness, etc. As herein amended, the Credit Agreement
          ------------------------------
shall remain in full force and effect and is hereby ratified and confirmed in
all respects.  After the Amendment Effective Date, all references in the Credit
Agreement and the other Loan Documents to the "Credit Agreement" or similar
terms shall refer to the Amended Agreement.

     4.2  Counterparts.  This Amendment may be executed in any number of
          ------------
counterparts and by the different parties on separate counterparts, and each
such counterpart shall be deemed to be an original but all such counterparts
shall together constitute one and the same agreement.

     4.3  Expenses.  The Company agrees to pay all reasonable expenses of the
          --------
U.S. Agent, including reasonable fees and charges of counsel for the U.S. Agent,
in connection with the preparation, execution and delivery of this Amendment.

     4.4  Governing Law.  This Amendment shall be construed in accordance with
          -------------
and governed by the substantive laws of the State of Illinois applicable to
contracts made and to be performed entirely within such State.

     4.5  Successors and Assigns.  This Amendment shall be binding upon Parent,
          ----------------------
the Company, the Banks and the Agents and their respective successors and
assigns, and shall inure to the benefit of Parent, the Company, the Banks and
the Agents and the respective successors and assigns of the Banks and the
Agents.

     4.6  Amendment to Intercreditor Agreement.  The Required Banks hereby
          ------------------------------------
direct the U.S. Agent to enter into an amendment to the Intercreditor Agreement
substantially in the form of Exhibit D hereto.
                             ---------
<PAGE>

     Delivered at Chicago, Illinois, as of the day and year first above written.

                                         UNITED RENTALS, INC.


                                         By__________________________________
                                           Chief Financial Officer


                                         UNITED RENTALS (NORTH AMERICA), INC.


                                         By__________________________________
                                           Chief Financial Officer


                                         BANK OF AMERICA, N.A., as U.S. Agent


                                         By__________________________________
                                         Title_______________________________


                                         BANK OF AMERICA, N.A., as a U.S. Bank,
                                         as Issuing Bank and as Swing Line Bank


                                         By__________________________________
                                         Title_______________________________


                                         BANK OF AUSTRIA CREDITANSTALT, as a
                                         U.S. Bank


                                         By__________________________________
                                         Title_______________________________
<PAGE>

                                         THE BANK OF NEW YORK, as a U.S. Bank

                                         By__________________________________
                                         Title_______________________________


                                         CREDIT LYONNAIS NEW YORK BRANCH, as a
                                         U.S. Bank and as a Documentation Agent


                                         By__________________________________
                                         Title_______________________________


                                         DEUTSCHE BANK AG, New York Branch
                                         and/or Cayman Islands Branch, as a U.S.
                                         Bank and as a Documentation Agent


                                         By__________________________________
                                         Title_______________________________


                                         By__________________________________
                                         Title_______________________________


                                         ALLFIRST BANK, as a U.S. Bank


                                         By__________________________________
                                         Title_______________________________


                                         SUMMIT BANK, as a U.S. Bank


                                         By__________________________________
                                         Title_______________________________


                                         NATIONAL CITY BANK, as a U.S. Bank

                                       2
<PAGE>


                                        By__________________________________
                                        Title_______________________________


                                        COMERICA BANK, as a U.S. Bank


                                        By__________________________________
                                        Title_______________________________


                                        FLEET NATIONAL BANK, N.A., as a U.S.
                                        Bank and as a Documentation Agent


                                        By__________________________________
                                        Title_______________________________


                                        HARRIS TRUST AND SAVINGS BANK,
                                         as a U.S. Bank


                                        By__________________________________
                                        Title_______________________________


                                        THE BANK OF NOVA SCOTIA, as a U.S. Bank


                                        By__________________________________
                                        Title_______________________________


                                       3
<PAGE>


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


                                        By__________________________________
                                        Title_______________________________


                                        CIBC INC., as a U.S. Bank


                                        By__________________________________
                                        Title_______________________________


                                        LASALLE BANK NATIONAL ASSOCIATION, as a
                                        U.S. Bank


                                        By__________________________________
                                        Title_______________________________


                                        CITICORP DEL-LEASE, INC., as a U.S. Bank


                                        By__________________________________
                                        Title_______________________________


                                        ERSTE BANK DER OESTERREICHISCHEN
                                        SPARKASSEN AG-NEW YORK, as a U.S. Bank


                                        By__________________________________
                                        Title_______________________________


                                        CITY NATIONAL BANK, as a U.S. Bank


                                        By__________________________________
                                        Title_______________________________

                                       4
<PAGE>

                                        THE FUJI BANK, LIMITED, as a U.S. Bank

                                        By__________________________________
                                        Title_______________________________


                                        BANKERS TRUST COMPANY, as a U.S. Bank

                                        By__________________________________
                                        Title_______________________________


                                        WELLS FARGO BANK, N.A., as a U.S. Bank

                                        By__________________________________
                                        Title_______________________________


                                        THE SAKURA BANK LTD., as a U.S. Bank


                                        By__________________________________
                                        Title_______________________________

                                       5
<PAGE>

                                        FIRSTRUST BANK, as a U.S. Bank


                                        By__________________________________
                                        Title_______________________________


                                        HUA NAN COMMERCIAL BANK, LTD., as a U.S.
                                        Bank


                                        By__________________________________
                                        Title_______________________________



                                        LAND BANK OF TAIWAN, as a U.S. Bank


                                        By__________________________________
                                        Title_______________________________

                                       6
<PAGE>

                                SCHEDULE 1.1(C)

                               PRICING SCHEDULE

     The Floating Rate Margin, the Fixed Rate Margin, the rate per annum
applicable for facility fees and the rate per annum applicable for letter of
credit fees for Financial Letters of Credit, Non-Financial Letters of Credit and
Commercial Letters of Credit, respectively, shall be determined in accordance
with the table below and the other provisions of this Schedule 1.1(C).
                                                      ---------------


<TABLE>
<CAPTION>
============================================================================
                        Level I   Level II   Level III   Level IV   Level V



----------------------------------------------------------------------------
<S>                     <C>       <C>        <C>         <C>        <C>
Rate for                 0.3750%    0.3750%     0.3750%    0.3750%   0.3000%
Facility Fee


----------------------------------------------------------------------------
Fixed Rate Margin*       1.8750%    1.6250%     1.3750%    1.2500%   1.2000%


----------------------------------------------------------------------------
Floating Rate Margin          0          0           0          0         0


----------------------------------------------------------------------------
Rate for                 0.7500%    0.6250%     0.5000%    0.4375%   0.4500%
Non-Financial LC Fee


----------------------------------------------------------------------------
Rate for                 1.8750%    1.6250%     1.3750%    1.2500%   1.2000%
Financial LC Fee*
----------------------------------------------------------------------------
</TABLE>

                                       7
<PAGE>

<TABLE>
<S>                      <C>         <C>        <C>        <C>       <C>
----------------------------------------------------------------------------
Rate for                 0.7500%    0.6250%     0.5000%    0.4375%   0.4500%
Commercial LC Fee


============================================================================
</TABLE>


     Level I applies when the Funded Debt to Cash Flow Ratio is equal to or
     -------
greater than 4.0 to 1.0.

     Level II applies when the Funded Debt to Cash Flow Ratio is equal to or
     --------
greater than 3.5 to 1.0 but less than 4.0 to 1.0.

     Level III applies when the Funded Debt to Cash Flow Ratio is equal to or
     ---------
greater than 3.0 to 1.0 but less than 3.5 to 1.0.

     Level IV applies when the Funded Debt to Cash Flow Ratio is equal to or
     --------
greater than 2.5 to 1.0 but less than 3.0 to 1.0.

     Level V applies when the Funded Debt to Cash Flow Ratio is less than 2.5 to
     -------
1.0.

     The applicable Level shall be adjusted, to the extent applicable, 45 days
(or, in the case of the last Fiscal Quarter of any Fiscal Year, 90 days) after
the end of each Fiscal Quarter based on the Funded Debt to Cash Flow Ratio as of
the last day of such Fiscal Quarter; provided that if the Company fails to
                                     --------
deliver the financial statements required by Section 10.1.1 or 10.1.2, as
                                             --------------    ------
applicable, and the related certificate required by Section 10.1.4 by the 45th
                                                    --------------
day (or, if applicable, the 90th day) after any Fiscal Quarter, Level I shall
apply until such financial statements are delivered.

     *    The Fixed Rate Margin and the Rate for Financial LC Fee for each of
          Level I through Level V shall be reduced by (i) 0.125% so long as the
          credit rating of the Company's senior unsecured non-credit-enhanced
          Debt (or an indicative rating for such Debt) is at least Ba2 by
          Moody's Investors Service, Inc. and BB+ by Standard & Poor's Ratings
          Group and (ii) an additional 0.125% so long as such credit rating is
          at least Ba1 by Moody's Investors Service, Inc. and BB+ by Standard &
          Poor's Ratings Group. The Company agrees to notify the U.S. Agent
          (which shall promptly notify each of the U.S. Banks) in writing within
          3 Business Days of any change in a credit rating which causes a change
          in the Fixed Rate Margin and the Rate for Financial LC Fee pursuant to
          the preceding sentence. Any change in the Fixed Rate Margin and the
          Rate for Financial LC Fee pursuant to this paragraph shall be
          effective on the third Business Day after the change in the Company's
          credit rating which causes any such change in the Fixed Rate Margin
          and the Rate for Financial LC Fee.
<PAGE>

                                   EXHIBIT A

                                 CONFIRMATION

                           Dated as of May 12, 2000


TO:  Bank of America, N.A., as U.S. Agent for the Banks party to the Credit
     Agreement referred to below, as Agent for the Lenders party to the
     Term Loan B Agreement referred to below, as Agent for the Lenders
     party to the Term Loan C Agreement referred to below, and as
     Collateral Agent under the Intercreditor Agreement referred to below;
     the Banks party to the Credit Agreement; and the Lenders party to the
     Term Loan B Agreement and the Term Loan C Agreement


     Reference is made to (a) the Credit Agreement dated as of September 29,
1998 (as previously amended, the "Credit Agreement") among United Rentals (North
                                  ----------------
America), Inc. (the "Company"), United Rentals, Inc. ("Parent"), United Rentals
                     -------                           ------
of Canada, Inc., various financial institutions, Bank of America Canada, as
Canadian Agent, and Bank of America, N.A., as U.S. Agent, (b) the Term Loan
Agreement dated as of July 10, 1998 and amended and restated as of May 12, 2000
(as so amended and restated, the "Term Loan B Agreement") among Parent, the
                                  ---------------------
Company, various financial institutions and Bank of America, N.A., as Agent, (c)
the Term Loan Agreement dated as of July 15, 1999 and amended and restated as of
May 12, 2000 (as so amended and restated, the "Term Loan C Agreement") among
                                               ---------------------
Parent, the Company, various financial institutions and Bank of America, N.A.,
as Agent, (d) the Fourth Amendment dated as of May 12, 2000 (the "Fourth
                                                                  ------
Amendment to Credit Agreement") to the Credit Agreement, (e) the Second Restated
-----------------------------
U.S. Guaranty dated as of September 29, 1998 executed by various Subsidiaries of
the Company, (f) the Second Restated U.S. Security Agreement dated as of
September 29, 1998 (the "Security Agreement") among the Company, Parent, various
                         ------------------
Subsidiaries of the Company and Bank of America, N.A., as Collateral Agent (in
such capacity, the "Collateral Agent"), (g) the Second Restated Company Pledge
                    ----------------
Agreement dated as of September 29, 1998 between the Company and the Collateral
Agent, (h) the Restated Parent Guaranty dated as of September 29, 1998 executed
by Parent, (i) the Restated Parent Pledge Agreement dated as of September 29,
1998 between Parent and the Collateral Agent, and (j) the Intercreditor
Agreement dated as of September 29, 1998 (the "Intercreditor Agreement") among
                                               -----------------------
Bank of America, N.A., as U.S. Agent, Bank of America, N.A., as Term Agent, Bank
of America, N.A. as agent for the Lenders under the Term Loan C Agreement, and
the Collateral Agent.  Each document referred to in items (e) through (i) above
                                                    ---------         ---
is called a "Credit Document".  Capitalized terms used but not defined herein
             ---------------
shall have the meanings set forth in the Credit Agreement.

     Each of the undersigned (a) confirms to each of the addressees that (i)
each Credit Document to which such undersigned is a party continues in full
force and effect on and after
<PAGE>

the date hereof, and is the legal, valid and binding obligation of such
undersigned, enforceable against such undersigned in accordance with its terms;
and (ii) the obligations and liabilities guaranteed or secured (as applicable)
under each Credit Document include, without limitation, all obligations and
liabilities of the Company and Parent under the Credit Agreement, as amended by
the Fourth Amendment to Credit Agreement (as so amended, the "Amended Credit
                                                              --------------
Agreement"), the Term Loan B Agreement and the Term Loan C Agreement; (b) agrees
---------
that each reference in each Credit Document to the "Credit Agreement" or any
                                                    ----------------
similar term shall, after the date hereof, be deemed to be a reference to the
Amended Credit Agreement; (c) agrees that each reference in each Credit Document
to the "Term Loan Agreement" or any similar term shall, after the date hereof,
        -------------------
be deemed to be a reference to the Term Loan B Agreement; and (d) agrees that
each reference in each Credit Document to "Permitted Senior Secured Debt" shall
                                           -----------------------------
include all Debt under the Term Loan C Agreement.
<PAGE>

     IN WITNESS WHEREOF, each of the undersigned has caused this Confirmation to
be executed and delivered by its duly authorized officer as of the date first
written above.


                                        UNITED RENTALS (NORTH AMERICA), INC.



                                        By:_____________________________________
                                        Title: Chief Financial Officer


                                        UNITED RENTALS, INC.



                                        By:_____________________________________
                                        Title:__________________________________



                                                                    Confirmation

                               Signature Page-1
<PAGE>

               ADVANCE BARRICADES AND SIGNING, INC.
               COAST LINE MARKING, INC.
               CONTRACTOR BARRICADE SERVICE, INC.
               FLASHER BARRICADES, INC.
               FLASHER CO. OF KANSAS, INC.
               FLASHER COMPANY OF OKLAHOMA, INC.
               FLASHER OF DIXIE II, INC.
               HIGHWAY SUPPLY COMPANY
               JADCO SIGNING, INC.
               LECTRIC LIGHTS COMPANY
               LIDDELL BROS., INC.
               LIDDELL MANAGEMENT CO., INC.
               PRECISION PAVEMENT MARKING, INC.
               SAFETY LIGHTS SALES AND LEASING, INC. OF TEXAS
               STATE BARRICADING, INC.
               STATE SIGN, INC.
               TRAFFIC SAFETY SERVICES, INC.
               TWO WAY TRAFFIC, INC.
               UNITED RENTALS GULF, INC.
               UNITED RENTALS NORTHWEST, INC.
               UNITED RENTALS SOUTHEAST, INC.
               UNITED RENTALS OF CANADA, INC.
               WARNING LITES OF MINNESOTA, INC.
               WARNING SAFETY LIGHTS, INC.
               WARNING SAFETY LIGHTS OF GEORGIA, INC.
               WLI INDUSTRIES, INC.
               WARNING LITES OF INDIANA, INC.
               WARNING LITES OF IOWA, INC.
               SAFE-T-FLARE SERVICES, INC.
               WORK SIGNS, INC.
               WORK ZONE, INC.
               WORK ZONE SAFETY, INC.
               WOUDENBERG ENTERPRISES, INC.
               WYNNE SYSTEMS, INC.


               By:______________________________
               Title:___________________________


                                                                    Confirmation

                               Signature Page-2
<PAGE>

                                                                    Confirmation

                               Signature Page-3
<PAGE>

                              UNITED EQUIPMENT RENTALS GULF, L.P.
                              By: United Rentals (North America), Inc.
                              Its: General Partner


                              By:________________________________
                              Title:_____________________________


                              UNITED RENTALS SOUTHEAST, L.P.
                              By: United Rentals (North America), Inc.
                              Its: General Partner


                              By:________________________________
                              Title:_____________________________




                                                                    Confirmation

                               Signature Page-3

<PAGE>

Accepted and Agreed to this
12th day of May, 2000


BANK OF AMERICA, N.A. (formerly Bank of America
National Trust and Savings Association), as Collateral Agent
for the Benefited Parties


By:_________________________
Title:______________________
          Agency Officer



                                                                    Confirmation

                               Signature Page-4
<PAGE>

                                   EXHIBIT D

                                    FORM OF
                     AMENDMENT TO INTERCREDITOR AGREEMENT

                           Dated as of May 12, 2000



          Reference is made to the Intercreditor Agreement (the "Intercreditor
                                                                 -------------
Agreement") dated as of September 29, 1998 among Bank of America, N.A. (formerly
---------
known as Bank of America National Trust and Savings Association), as U.S. Agent,
Bank of America, N.A., as Term Agent, various other senior secured creditors of
the Company and Bank of America, N.A., as Collateral Agent.  Capitalized terms
not defined herein are used as defined in the Intercreditor Agreement.

          Each of the undersigned hereby agrees that subsection 3(f) of the
Intercreditor Agreement is amended in its entirety to read as follows:

          "(f) The Collateral Agent may (and shall at the request of any
          Grantor), without the approval of any other Benefited Party, (x)
          release any Collateral under any Security Document which is permitted
          to be sold or disposed of or otherwise released pursuant to the Credit
          Agreement, the Term Loan Agreement and any applicable Permitted Senior
          Secured Debt Agreement and execute and deliver such releases as may be
          necessary to terminate of record the Collateral Agent's security
          interest (for the benefit of the Benefited Parties) in such
          Collateral, (y) release any Grantor from its obligations under the
          U.S. Guaranty which is permitted to be released pursuant to the Credit
          Agreement, the Term Loan Agreement and any applicable Permitted Senior
          Secured Debt Agreement and (z) subordinate its interest in any
          Collateral to the holder of any Lien on such Collateral which is
          permitted by clause (d) or (h) of Section 10.8 of the Credit
          Agreement, clause (d) or (h) of Section 10.8 of the Term Loan
          Agreement and the comparable provisions of each Permitted Senior
          Secured Debt Agreement. In determining whether any such release or
          subordination is permitted, the Collateral Agent may (I) rely, as to
          factual matters, on a Certificate from the Company and (II) may (but
          shall not be obligated to) seek (and, if obtained, rely upon)
          instructions from the Required Banks (as to whether any such release
          is permitted under the Credit Agreement), from the Required Term
          Lenders (as to whether any such release is permitted under the Term
          Loan Agreement) and from the Required Permitted Creditors under each
          Permitted Senior Secured Debt Agreement or group of related Permitted
          Senior Secured Debt Agreements (as to whether any such release is
          permitted under the applicable Agreement or Agreements). In addition,
          the Collateral Agent may release all Collateral upon

                                                                    Confirmation
<PAGE>

          receipt of (i) written notice from the U.S. Agent that all Credit
          Obligations have been paid in full (other than contingent obligations
          (A) in respect of Letters of Credit which have been cash
          collateralized or otherwise provided for to the satisfaction of the
          Issuing Bank (as defined in the Credit Agreement) and (B) arising
          under provisions of the Credit Agreement which by their terms survive
          termination thereof) and all commitments to create Credit Obligations
          have been terminated, (ii) written notice from the Term Agent that all
          Term Obligations have been paid in full (other than any Term
          Obligations arising under provisions of the Term Loan Agreement which
          by their terms survive termination thereof), (iii) written notice from
          each holder of Hedging Obligations that such Hedging Obligations have
          been paid in full or otherwise provided for to the satisfaction of
          such holder (it being understood that if the Collateral Agent notifies
          the Banks, the Term Lenders and the Permitted Creditors that it
          intends to release the Collateral pursuant to this sentence, the
          Collateral Agent may conclusively presume that there are no Hedging
          Obligations other than those of which the Collateral Agent receives
          notice not later than the later of five business days after notice is
          sent by the Collateral Agent of such intended release and the business
          day before the date on which such release occurs), (iv) written notice
          from each Permitted Creditor (or, in the case of any group of
          Permitted Creditors, an agent therefor) that all Permitted Senior
          Secured Debt Obligations owed to such Permitted Creditor (or group of
          Permitted Creditors) have been paid in full (other than any Permitted
          Senior Secured Debt Obligations arising under provisions of any
          Permitted Senior Secured Debt Document which by their terms survive
          termination thereof) and (v) payment of all other Benefited
          Obligations owed to the Collateral Agent or of which the Collateral
          Agent has received written notice."

          IN WITNESS WHEREOF, each of the undersigned has caused this amendment
to be executed and delivered by its duly authorized representative as of the
date hereof.

                                  Signature-2
<PAGE>

                    BANK OF AMERICA, N.A., as Collateral Agent
                    By:
                    Name:
                    Title:


                    BANK OF AMERICA, N.A., as U.S. Agent
                    By:
                    Name:
                    Title:


                    BANK OF AMERICA, N.A., as Term Agent
                    By:
                    Name:
                    Title:


                    BANK OF AMERICA, N.A., as Agent for the holders of the
                    Permitted Senior Secured Debt

                    By:
                    Name:
                    Title:

                                  Signature-3