<PAGE>   1
                                                                 Exhibit 10.4(h)


                      FOURTH AMENDMENT TO CREDIT AGREEMENT

         This Amendment ("Amendment") is made as of the 16th day of March, 1998
by and between BOWMAR INSTRUMENT CORPORATION, an Indiana corporation (the
"Borrower") and Bank One, Arizona, NA (the "Bank").

         WHEREAS, the Borrower and the Bank entered into a Loan Agreement, dated
August 28, 1995 as amended (if applicable) (the "Credit Agreement"); and

         WHEREAS, the parties hereto desire to amend the Credit Agreement as set
forth below:

         NOW, THEREFORE, the parties hereto agree as follows:

         1. Capitalized terms not defined herein shall have the meaning ascribed
in the Credit Agreement.

         2. The definition of "RLC Maturity Date" on the eighth page of the Loan
         Agreement is hereby amended in its entirety to read as follows:

                           "RLC Maturity Date" means February 28, 2000.

         3. Sections 10.8 (a), (c) and (e) are hereby amended in their entirety
         to read as follows:

         (a) The Current Ratio to be less than 2.0 to 1.0 at the end of any
         quarterly accounting period of Borrower.

         (c) Its Tangible Net Worth to be less than $7,600,000.00 at the end of
         any quarterly accounting period of Borrower.

         (e) Its Debt Coverage Ratio to be less than 2.3 to 1.0 from 3-31-98
         thru 6-30-98, from 6-30-98 and thereafter the Debt Coverage Ratio to be
         less than 2.5 to 1.0 at the end of any quarterly accounting period of
         Borrower.

         4. Section 10.8 (b) is hereby deleted in its entirety.

         5. The Borrower represents and warrants that (a) the representations
and warranties contained in the Credit Agreement are true and correct in all
material respects as of the date of this Amendment, (b) no condition, act or
event which could constitute an Event of Default under the Credit Agreement
exists, and (c) no condition, event, act or omission has occurred, which, with
the giving of notice or passage of time, would constitute an Event of Default
under the Credit Agreement.

         6. The Borrower agrees to pay all fees and out-of-pocket disbursements
incurred by the Bank in connection with this Amendment, including legal fees
incurred by the Bank in the preparation, consummation, administration and
enforcement of this Amendment.

         7. This Amendment shall become effective only after it is fully
executed by the Borrower and the Bank and the Bank shall have received from the
Borrower the following documents: (i) Business Loan Application, and (ii)
Promissory Note Modification Agreement. Except as amended by this Amendment, the
Credit Agreement shall remain in full force and effect in accordance with its
terms.

         8. This Amendment is a modification only and not a novation. Except for
the above-quoted modification(s), the Credit Agreement, any agreement or
security document, and all the terms and conditions thereof, shall be and remain
in full force and effect with the changes herein deemed to be incorporated
therein. This Amendment is to be considered attached to the Credit Agreement and
made a part thereof. This Amendment shall not release or affect the liability of
any guarantor, surety or endorser of the Credit Agreement or release any owner
of collateral securing the Credit Agreement. The validity, priority and
enforceability of the Credit Agreement shall not be impaired hereby. To the
extent that any provision of this Amendment conflicts with any term or condition
set forth in the Credit Agreement, or any agreement or security document
executed in conjunction therewith, the provisions of this Amendment shall
supersede and control. Borrower acknowledges that as of the date of this
Amendment it has no offsets with respect to all amounts owed by Borrower to Bank
and Borrower waives and releases all claims which it may have against Bank
arising under the Agreement on or prior to the date of this Amendment.


                                       1
<PAGE>   2
         9. The Borrower acknowledges and agrees that this Amendment is limited
   to the terms outlined above, and shall not be construed as an amendment of
   any other terms or provisions of the Agreement; The Borrower hereby
   specifically ratifies and affirms the terms and provisions of the Agreement.
   Borrower releases Bank from any and all claims which may have arisen, known
   or unknown, in connection with the Credit Agreement on or prior to the date
   hereof. This Amendment shall not establish a course of dealing or be
   construed as evidence of any willingness on the Lender's part to grant other
   or future amendments, should any be requested.

         IN WITNESS WHEREOF, the parties have entered into this Amendment as of
the day and year first above written.

                                                              BORROWER:

BANK ONE, ARIZONA, NA                        BOWMAR INSTRUMENT CORPORATION,
                                             an Indiana corporation




By: /s/ Chad Christian                      By: /s/ Joseph G. Warren, Jr.
   -------------------                          ---------------------------
Name: Chad Christian                         Name:    Joseph G. Warren, Jr.
Title: Vice President                        Title:   Vice President







                                       2