Document:

<PAGE>   1
                                                                     Exhibit 4.3

                                                                  EXECUTION COPY

                             PAINE WEBBER GROUP INC.

                                       to

                 CHASE MANHATTAN BANK USA, NATIONAL ASSOCIATION

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

                          Subordinated Debt Securities

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

                          Fourth Supplemental Indenture

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

                          Dated as of November 3, 2000

                           Supplementing the Indenture
                           Dated as of March 15, 1988

                                 ---------------
<PAGE>   2
                                    FOURTH SUPPLEMENTAL INDENTURE dated as of
                           November 3, 2000, among PAINE WEBBER GROUP INC., a
                           corporation duly organized and existing under the
                           laws of Delaware (herein called the "Company"),
                           having its principal office at 1285 Avenue of the
                           Americas, New York, New York 10019, UBS AMERICAS
                           INC., a corporation duly organized and existing under
                           the laws of Delaware (herein called "UBS Americas"),
                           having its principal office at 677 Washington
                           Boulevard, Stamford, Connecticut 06901, and CHASE
                           MANHATTAN BANK USA, NATIONAL ASSOCIATION, a
                           corporation duly organized and existing under the
                           laws of the State of Delaware (formerly known as
                           Chemical Bank (Delaware)), as Trustee (herein called
                           the "Trustee").

                                    RECITALS

                  The Company and the Trustee are parties to an Indenture dated
as of March 15, 1988, as supplemented by the First Supplemental Indenture dated
as of September 22, 1989, the Second Supplemental Indenture dated as of March
22, 1991 and the Third Supplemental Indenture dated as of November 30, 1993 (as
so supplemented, the "Indenture"), relating to the issuance from time to time by
the Company of its Securities. Capitalized terms used, and not otherwise
defined, herein shall have the same meanings given them in the Indenture.

                  The Company, UBS AG, an Aktiengesellschaft organized under the
laws of Switzerland, and UBS Americas (then known as Neptune Merger Subsidiary,
Inc.), a wholly owned subsidiary of UBS AG have entered into an Agreement and
Plan of Merger, dated as of July 12, 2000 (the "Merger Agreement"). Pursuant to
the Merger Agreement, the Company will merge with and into UBS Americas, with
UBS Americas as the surviving corporation (the "Merger"), the Merger to be
effective upon the filing, in the office of the Secretary of State of the State
of Delaware, of a certificate of merger the ("Effective Time").

                  The Company has requested the Trustee to join with it in the
execution and delivery of this fourth supplemental indenture (this "Fourth
Supplemental Indenture") in order to supplement and amend the Indenture, by
amending and adding certain provisions thereof, pursuant to which UBS Americas
will expressly assume the due and punctual payment of the principal of (and
premium, if any) and interest on
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all Securities and the performance of every covenant of the Indenture on the
part of the Company to be performed or observed (the "Assumption"), as set forth
in Section 801(1) of the Indenture.

                  Section 801 of the Indenture provides that the Assumption must
be effected through a supplemental indenture, executed and delivered to the
Trustee, in form satisfactory to the Trustee.

                  Section 901 of the Indenture provides that a supplemental
indenture may be entered into by the Company and the Trustee, without the
consent of any Holders, to evidence the succession of another corporation to the
Company and assumption by any such successor of the covenants of the Company
contained in the Indenture and in the Securities.

                  At the request of the Trustee, the Company (a) has furnished
the Trustee with (1) an Officers' Certificate complying with the requirements of
Sections 102 and 801(4) of the Indenture, stating, among other things, that the
Merger and this Fourth Supplemental Indenture comply with Article Eight of the
Indenture and that all conditions precedent provided for in the Indenture
relating to the Merger and this Fourth Supplemental Indenture have been complied
with and (2) an Opinion of Counsel complying with the requirements of Sections
102, 801(4) and 903 of the Indenture and (b) has delivered to the Trustee a
Board Resolution as required by Section 901 of the Indenture authorizing the
execution by the Company of this Fourth Supplemental Indenture and its delivery
by the Company to the Trustee.

                  At the request of the Trustee, UBS Americas (a) has furnished
the Trustee with (1) an opinion of counsel and (2) a Secretary's Certificate and
(b) has delivered to the Trustee a Board Resolution authorizing the execution by
UBS Americas of this Fourth Supplemental Indenture and its delivery by UBS
Americas to the Trustee.

                  All things necessary to make this Fourth Supplemental
Indenture a valid agreement of the Company, UBS Americas and the Trustee, in
accordance with the terms of the Indenture, and a valid amendment of and
supplement to the Indenture have been done.

                  NOW, THEREFORE, THIS FOURTH SUPPLEMENTAL INDENTURE WITNESSETH:

                  For and in consideration of the premises and the purchase of
the Securities by the Holders thereof, it is mutually agreed, for the equal and
proportionate benefit of all Holders of the Securities or of series thereof, as
follows:
<PAGE>   4
I.       AMENDMENTS TO THE INDENTURE

                  A. Assumption and Substitution. UBS Americas hereby assumes,
as of the Effective Time, the due and punctual payment of the principal of (and
premium, if any) and interest on all the Securities and the performance of every
covenant of the Indenture on the part of the Company to be performed or
observed. UBS Americas hereby succeeds to, and is substituted for, the Company
in the Indenture, as of the Effective Time.

II.      NOTICES

                  A. For purposes of Section 105(2) of the Indenture, the
address of UBS Americas is: 677 Washington Boulevard, Stamford, Connecticut
06901, Attention: Secretary.

                  B. UBS Americas hereby agrees to give the Trustee prompt
notice of the Effective Time.

III.     NOTATION ON SECURITIES

                  A. All Securities authenticated and delivered after the
Effective Time shall bear the following notation, which may be stamped or
imprinted thereon:

                           "In connection with the merger of Paine Webber Group
                  Inc. (the "Company") into UBS Americas Inc. and pursuant to
                  the Fourth Supplemental Indenture dated as of November 3,
                  2000, UBS Americas Inc. has assumed the due and punctual
                  payment of the principal of (and premium, if any) and interest
                  on this Security and the performance of every covenant of the
                  Indenture on the part of the Company to be performed or
                  observed."

                  UBS Americas shall arrange for and pay all expenses related to
such notation.

IV.      GENERAL PROVISIONS

                  A. The recitals contained herein shall be taken as the
statements of the Company and UBS Americas, and the Trustee assumes no
responsibility for the correctness of same. The Trustee makes no representation
as to the validity of this Fourth Supplemental Indenture. The Indenture, as
supplemented and amended by this Fourth Supplemental Indenture, is in all
respects hereby adopted, ratified and confirmed.

                  B. The Trustee hereby accepts this Fourth Supplemental
Indenture and agrees to perform the same under the terms and conditions set
forth in the Indenture.
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                  C. This instrument may be executed in any number of
counterparts, each of which so executed shall be deemed to be an original, but
all such counterparts shall together constitute but one and the same instrument.

                  D. This Fourth Supplemental Indenture shall be governed by and
construed in accordance with the laws of the State of New York.
<PAGE>   6
                  IN WITNESS WHEREOF, the parties hereto have caused this Third
Supplemental Indenture to be duly executed, and their corporate seals to be
hereunto affixed and attested, all as of the day and year first above written.

                                       PAINE WEBBER GROUP INC.,

                                            by  /s/ Authorized Officer
                                               ---------------------------------
[SEAL]                                         Title: Senior Vice President

Attest:

 /s/
---------------------------

                                       UBS AMERICAS INC.,

                                            by  /s/  Louis Eber
                                               ---------------------------------
                                               Title: Executive Director

[SEAL]                                      by  /s/  Sandra Ward Costin
                                               ---------------------------------
                                               Title: Director

Attest:

                                       THE CHASE MANHATTAN BANK, as Trustee,

                                            by  /s/ Authorized Officer
                                               ---------------------------------
                                               Title: Vice President
---------------------------

[SEAL]

Attest:<PAGE>   1

                                                                   EXHIBIT 10(s)

                        FIRST AMENDMENT TO LOAN DOCUMENTS

         This First Amendment to Loan Documents is made on the 9th day of June,
2000, by and between OLD KENT BANK, a Michigan banking corporation (the "Bank"),
One Vandenberg Center, Grand Rapids, Michigan 49503, and RIVIERA TOOL COMPANY, a
Michigan corporation ("Borrower"), 5460 Executive Parkway, Grand Rapids,
Michigan 49512.

                                    RECITALS:

         A.    The Bank and the Borrower are parties to a First Restated Loan
Agreement dated as of August 31, 1999 (the "Loan Agreement"), pursuant to which
the Bank has agreed, subject to the terms and conditions set forth in the Loan
Agreement and the Loan Documents, to extend to the Borrower certain credit
facilities, including but not limited to a Revolving L/C Loan in the maximum
principal amount of $10,000,000. Capitalized terms used but not defined in this
First Amendment shall have the meanings given such terms in the Loan Agreement.

         B.    The Revolving L/C Loan is evidenced by a Promissory Note in
substantially the form attached to the Loan Agreement as Exhibit A (the
"Revolving L/C Loan Note").

         C.    The Bank and the Borrower agreed, effective as of February 29,
2000, to extend the maturity date of the Revolving L/C Loan to September 1, 2001
from December 1, 2000, and the Bank and the Borrower wish to confirm such
agreement in writing.

         D.    At the Borrower's request, the Bank has agreed to extend to the
Borrower, in addition to the credit facilities currently outstanding under the
Loan Agreement, a non-revolving line of credit loan in the maximum principal
amount of $1,000,000.00 (the "2000 Equipment L/C Loan") for the purpose of
financing the Borrower's acquisition of certain equipment.

         E.    The 2000 Equipment L/C Loan shall be evidenced by a Promissory
Note in substantially the form attached to this First Amendment as Exhibit A
(the "2000 Equipment L/C Note").

         F.    The Bank and the Borrower wish to amend the Loan Agreement to
reflect their agreement that the 2000 Equipment L/C Note shall be deemed to have
been issued pursuant to the Loan Agreement and that the 2000 Equipment L/C Loan
shall be deemed Indebtedness as defined in the Loan Agreement, and to reflect
the other agreements set forth below.

         NOW, THEREFORE, the Bank and the Borrower agree as follows:

         1.    Extension of Maturity Date of Revolving L/C Loan. Effective as of
February 29, 2000, the maturity date of the Revolving L/C Loan was extended to
September 1, 2001 from December 1, 2000.

         2.    2000 Equipment L/C Loan. The 2000 Equipment L/C Note shall be
deemed to have been issued pursuant to the Loan Agreement and shall constitute a
"Note", as defined therein, and the 2000 Equipment L/C Loan shall be deemed
Indebtedness as defined in the Loan Agreement. All of the collateral that
secures the Borrower's Indebtedness and obligations under the Loan Agreement
shall secure the 2000 Equipment L/C Loan to full extent and with the same
priority as such collateral secures the Indebtedness evidenced by the other
Notes currently outstanding under the Loan Agreement.

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         3.    Cross-Default. An Event of Default under the Loan Agreement shall
be a default under the 2000 Equipment L/C Note, and a default under the 2000
Equipment L/C Note shall be an Event of Default under the Loan Agreement and all
of the other Loan Documents, and shall entitle the Bank to pursue any remedies
available to it under the Loan Agreement, any of the security agreements or
other collateral security documents referred to therein, or otherwise available
to the Bank at law or in equity or under any other agreement between the Bank
and the Borrower.

         4.    Special Terms Relating to 2000 Equipment L/C Loan. Advances of
the 2000 Equipment L/C Loan shall be conditioned upon the Borrower's compliance
with all of the terms and conditions of the Loan Agreement, as amended hereby,
and shall be subject to all limitations set forth in the Loan Agreement, as
amended hereby, including the following:

               (a) Under no circumstances shall the Bank have any obligation to
         make principal advances of the 2000 Equipment L/C Loan after the
         "Amortization Commencement Date", as defined in the 2000 Equipment L/C
         Loan Note.

               (b) The Borrower acknowledges that the Bank shall at no time ever
         be obligated to advance to the Borrower more than a total of
         $1,000,000.00 in principal under the 2000 Equipment L/C Loan, and that
         the 2000 Equipment L/C Loan is not a revolving loan. Once repaid,
         amounts borrowed under the 2000 Equipment L/C Loan may not be
         re-borrowed.

               (c) The principal amount of each 2000 Equipment L/C Loan advance
         shall not exceed an amount equal to eighty percent (80%) of the
         Borrower's actual cost for the equipment that will be purchased with
         the proceeds of such advance (excluding tax, insurance, delivery and
         set-up charges, the cost of maintenance contracts, and any other costs
         not paid or payable by the Borrower to the equipment manufacturer as a
         direct cost of acquiring the equipment).

               (d) The proceeds of each 2000 Equipment L/C Loan advance shall be
         advanced by the Bank to the Borrower only after (i) receipt by the Bank
         of such security agreements and financing statements (or supplements or
         amendments thereto) covering the equipment, duly executed by the
         Borrower, as the Bank may reasonably require to create and perfect its
         security interest therein, (ii) receipt by the Bank of such other
         documents as the Bank may reasonably require to create and perfect its
         security interest therein, together with such evidence as the Bank may
         reasonably require in order to establish the cost of such equipment to
         the Borrower and that the equipment is completed and ready for shipment
         to the Borrower, and (iii) the Borrower's compliance with the
         conditions described in Section 10 of the Loan Agreement.

         5.    Effect of this First Amendment. The Loan Documents are hereby
amended as required (and only as required) to give effect to the amendments
expressly provided for in this First Amendment. Each and every one of the Loan
Documents shall be deemed to have been amended by this First Amendment as if
such Loan Documents had been specifically amended by separate instrument. This
First Amendment shall be a Loan Document, and all references in any Loan
Document to the "Loan Documents" shall refer to the Loan Documents, as amended
hereby.

         6.    No other Amendments, etc. Except as expressly set forth above,
the Loan Agreement, each of the Notes, and all of the other Loan Documents shall
remain in full force and effect as originally executed and delivered by the
parties, and are hereby ratified and affirmed by the undersigned.

         7.    Counterparts. This First Amendment may be executed in one or more
counterparts, each of which shall be considered an original and all of which
shall constitute the same instrument.

         IN WITNESS WHEREOF, the parties have executed this First Amendment to
Loan Documents as of the day and year first above written.

                                       2
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WITNESSES:

                                         RIVIERA TOOL COMPANY

/s/ Peter C. Canepa                      By:  /s/ Kenneth K. Rieth
-------------------                          -----------------------------------
                                             Kenneth R. Rieth, President
/s/ Marcia Shumate
------------------

                                         OLD KENT BANK

/s/ Peter C. Canepa                      By:  /s/ Gilbert A. Segovia
-------------------                         ------------------------------------
                                            Gilbert A. Segovia, Vice President
/s/ Marcia Shumate
------------------

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