<PAGE>
                                                                  EXHIBIT 4.6(a)


                                 AMENDMENT NO. 4
                          DATED AS OF DECEMBER 14, 2004
                                    TO OFFER
                          DATED AS OF DECEMBER 3, 2001


          Amendment No. 4 dated as of December 14, 2004 (the "Amendment")
between IDEX CORPORATION (the "Borrower") and CALYON NEW YORK BRANCH (the
"Lender") to the Offer dated as of December 3, 2001, as amended (the "Offer"),
for an uncommitted line of credit by the Lender in favor of the Borrower.

     WHEREAS, the Borrower has requested that the Lender amend the Offer by
extending the Expiration Date and that the Offer be modified in certain other
respects, and

          WHEREAS, the Lender is willing to amend the Offer and grant such
request on and subject to the terms and conditions hereinafter set forth.

     NOW, THEREFORE, IT IS AGREED:

          SECTION 1. Capitalized Terms. All terms used but not otherwise defined
herein shall have the meaning ascribed to them in the Offer.

          SECTION 2. Amendments to Offer. The Offer is, effective as of the date
hereof and subject to the satisfaction of the conditions precedent set forth in
Section 3 below, hereby amended as follows:

     (i) The definition of "Credit Agreement" in Section 2 of the Offer is
hereby amended to read as follows:

               "the Credit Agreement dated December 14, 2004, among Borrower,
               Bank of America, N.A., as Agent, Swing Line Lender and L/C
               Issuer, Wachovia Bank, National Association, as Syndication
               Agent, and the other lenders party thereto."

     (ii) The definition of "Expiration Date" in Section 1 of the Offer is
hereby amended in clause (i) to state: "December 13, 2005."

     (iii) Section 11 of the Offer is hereby amended as follows: (a) in clause
(i) thereof, by replacing references to Sections 7.01, 7.02 and 7.03 with
references to Sections 6.01, 6.02 and 6.03, respectively; (b) in clause (iii)
thereof, by replacing references to Sections 8.03 and 8.02 with references to
Sections 7.03 and 7.02, respectively; (c) in clause (iv) thereof, by replacing
the reference to Section 8.02 with a reference to Section 7.02; and (d) in
clause (v) thereof, by replacing the reference to Section 8.03 with a reference
to Section 7.03.

          SECTION 3. Conditions to Effectiveness. This Amendment shall become
effective as of the date first written above when the Lender shall have received
(i) counterparts of this Amendment executed by the Borrower and the Lender, (ii)
a new Promissory Note (the "New Note") in favor of the Lender in the form
annexed hereto as Exhibit A in replacement of the Promissory Note dated May 21,
2004 in the principal amount of $30,000,000 (the "Old Note"), and (iii) such
other documents, instruments or agreement as the Lender shall reasonably
request. Upon its receipt of the New Note, duly executed by the Borrower, the
Lender shall return the Old Note to the Borrower marked "canceled."

          SECTION 4. Representations and Warranties. The Borrower hereby
represents and warrants that as of the date of effectiveness of this Amendment,
all representations and warranties set forth in the Offer are true and correct
as of such date, with each reference therein to the Offer meaning a reference to
the Offer as amended hereby.


                                      -21-

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          SECTION 5. Reference to and Effect on Credit Documents.

          (a) Upon the effectiveness hereof, on and after the date hereof, each
reference in the Credit Documents to "this Offer". "hereunder", "hereof" or
words of like import referring to the Offer and each reference in instruments
and documents delivered in connection therewith to "the Offer", "thereunder",
"thereof" or words of like import referring to the Offer shall mean and be a
reference to the Offer, as amended hereby.

          (b) Except as expressly modified hereby, the terms and provisions of
the Offer and each Credit Document shall remain in full force and effect and is
hereby ratified in all respects by the Borrower.

          (c) The execution, delivery and effectiveness of this Amendment shall
neither operate as a waiver of any rights, power or remedy of the Lender under
any of the Credit Documents nor constitute a waiver of any provision of any of
the Credit Documents.

          SECTION 6. Governing Law. This Amendment shall be governed by and
construed in accordance with the laws of the State of New York, without regard
to its conflicts of laws principles.

          SECTION 7. Execution in 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 to be
an original and all of which taken together shall constitute but one and the
same agreement. Delivery of an executed counterpart of a signature page to this
Amendment by telecopier shall be effective as delivery of a manually executed
counterpart of this Amendment.

                             [SIGNATURES TO FOLLOW]

          IN WITNESS WHEREOF, the parties hereto through their duly authorized
representatives have set their hand as of the date first written above.


                                                 IDEX CORPORATION


                                                 By:
                                                    ----------------------------
                                                 Name:
                                                 Title:



                                                 CALYON NEW YORK BRANCH


                                                 By:
                                                    ----------------------------
                                                 Name:
                                                 Title:


                                                 By:
                                                    ----------------------------
                                                 Name:
                                                 Title:


                                      -22-
<PAGE>


                                                                       EXHIBIT A

                                      NOTE

                                                              New York, New York


$30,000,000                     December 14, 2004



          For value received, IDEX CORPORATION, a Delaware corporation
("Borrower"), promises to pay to the order of CALYON NEW YORK BRANCH ("Lender")
the lesser of (a) Thirty Million United States Dollars ($30,000,000) and (b) the
aggregate unpaid principal amount of the Loans made by Lender to Borrower
pursuant to the Offer (as hereinafter defined) on the dates provided for
therein. Borrower promises to pay interest on the unpaid principal amount of
each such Loan on the dates and at the rate or rates provided for in the Offer.
All such payments of principal and interest shall be made in lawful money of the
United States in Federal or other immediately available funds at the office of
Calyon New York Branch, 1301 Avenue of the Americas, New York, NY 10019.

          All Loans made by Lender, the respective types and maturities thereof
and all repayments of the principal thereof shall be recorded by Lender and, if
Lender so elects in connection with any transfer or enforcement hereof,
appropriate notations to evidence the foregoing information with respect to each
such Loan then outstanding may be endorsed by Lender in the schedule attached
hereto or on a continuation of such schedule attached to and made a part hereof;
provided, however, that the failure of Lender to make any such recordation or
endorsement shall not affect the obligation of Borrower hereunder or under the
Offer.

          This note is the Note referred to in the Offer, dated as of December
3, 2001, between Borrower and Lender (as amended, and as the same may be
amended, supplemented or otherwise modified from time to time, the "Offer").
Terms defined in the Offer are used herein with the same meanings. Reference is
made to the provisions of the Offer for, among other things, prepayment of the
Loans and the acceleration of the maturity thereof.

         This Note is issued in substitution for, but not in repayment of, the
Note dated May 21, 2004. Any Loans outstanding under such prior Note shall
constitute loans under, evidenced by and subject to the terms of this Note.

          This Note shall be governed by and construed in accordance with the
laws of the State of New York.

                                                 IDEX CORPORATION


                                                 By:
                                                    ----------------------------
                                                 Name:
                                                 Title:



                                      -23-