
<PAGE>   1
                                                                EXHIBIT 10(a)(4)

                                   GUARANTY

                                          BRYAN                ,       TX
                                         ---------------------------------------
                                                   (City)             (State)

                                                    JUNE 30, 1996
                                               ---------------------------------

  For good and valuable consideration, the receipt and sufficiency of which are
hereby acknowledged, and to induce THE FIRST NATIONAL BANK OF BRYAN, 2807 TEXAS
AVE., P.O. BOX 833, BRYAN, TX 77805 (herein, with its participants, successors
and assigns, called "Lender"), at its option, at any time or from time to time
to make loans or extend other accommodations to or for the account of BODYBILT
SEATING INC. (herein called "Borrower") or to engage in any other transactions
with Borrower, the Undersigned hereby absolutely and unconditionally guarantees
to Lender the full and prompt payment when due, whether at maturity or earlier
by reason of acceleration or otherwise, of the debts, liabilities and
obligations described as follows:

    A. If this [ ] is checked, the Undersigned guarantees to Lender the payment
       and performance of the debt, liability or obligation of Borrower to
       Lender evidenced by or arising out of the
       following:________________________________ and any extensions, renewals
       or replacements thereof (hereinafter referred to as the "Indebtedness").

    B. If this [xx] is checked, the Undersigned guarantees to Lender the
       payment and performance of each and every debt, liability and
       obligation of every type and description which Borrower may now or at
       any time hereafter owe to Lender (whether such debt, liability or
       obligation now exists or is hereafter created or incurred, and whether
       it is or may be direct or indirect, due or to become due, absolute or
       contingent, primary or secondary, liquidated or unliquidated, or joint,
       several, or joint and several; all such debts, liabilities and
       obligations being hereinafter collectively referred to as the
       "Indebtedness").  Without limitation, this guaranty includes the
       following described debt(s):  PROMISSORY NOTE NUMBER 033-118-76 DATED
       6-30-96.

  The term "Indebtedness" as used in this guaranty shall not include any
  obligations entered into between Borrower and Lender after the date hereof 
  (including any extensions, renewals or replacements of such obligations) for 
  which Borrower meets the Lender's standard of creditworthiness based on 
  Borrower's own assets and income without the addition of a guaranty, or for 
  which a guaranty is required but Borrower chooses someone other than the 
  joint Undersigned to guaranty the obligation.

  The Undersigned further acknowledges and agrees with Lender that:

  1. No act or thing need occur to establish the liability of the Undersigned
hereunder, and no act or thing, except full payment and discharge of all
Indebtedness, shall in any way exonerate the Undersigned or modify, reduce,
limit or release the liability of the Undersigned hereunder.

  2. This is an absolute, unconditional and continuing guaranty of payment of
the Indebtedness and shall continue to be in force and be binding upon the
Undersigned, whether or not all Indebtedness is paid in full, until this
guaranty is revoked by written notice actually received by the Lender, and such
revocation shall not be effective as to Indebtedness existing or committed for
at the time of actual receipt of such notice by the Lender, or as to any
renewals, extensions and refinancings thereof.  If there be more than one
Undersigned, such revocation shall be effective only as to the one so revoking. 
The death or incompetence of the Undersigned shall not revoke this guaranty,
except upon actual receipt of written notice thereof by Lender and then only as
to the decedent or the incompetent and only prospectively, as to future
transactions, as herein set forth.

  3. If the Undersigned shall be dissolved, shall die, or shall be or become
insolvent (however defined) or revoke this guaranty, then the Lender shall have
the right to declare immediately due and payable, and the Undersigned will
forthwith pay to the Lender, the full amount of all Indebtedness, whether due
and payable or unmatured.  If the Undersigned voluntarily commences or there is
commenced involuntarily against the Undersigned a case under the United States
Bankruptcy Code, the full amount of all Indebtedness, whether due and payable or
unmatured, shall be immediately due and payable without demand or notice
thereof.

  4. The liability of the Undersigned hereunder shall be limited to a principal
amount of $ UNLIMITED (If unlimited or if no amount is stated, the Undersigned
shall be liable for all Indebtedness, without any limitation as to amount),
plus accrued interest thereon and all attorneys' fees, collection costs and
enforcement expenses referable thereto.  Indebtedness may be created and
continued in any amount, whether or not in excess of such principal amount,
without affecting or impairing the liability of the Undersigned hereunder.  The
Lender may apply any sums received by or available to Lender on account of the
Indebtedness from Borrower or any other person (except the Undersigned), from
their properties, out of any collateral security or from any other source to
payment of the excess.  Such application of receipts shall not reduce, affect
or impair the liability of the Undersigned hereunder.  If the liability of the
Undersigned is limited to a stated amount pursuant to this paragraph 4, any
payment made by the Undersigned under this guaranty shall be effective to
reduce or discharge such liability only if accompanied by a written transmittal
document, received by the Lender, advising the Lender that such payment is made
under this guaranty for such purpose.

  5. The Undersigned will pay or reimburse Lender for all costs and expenses
(including reasonable attorneys' fees and legal expenses) incurred by Lender in
connection with the protection, defense or enforcement of this guaranty in any
litigation or bankruptcy or insolvency proceedings.

THIS GUARANTY INCLUDES THE ADDITIONAL PROVISIONS ON PAGE 2, ALL OF WHICH ARE
MADE A PART HEREOF.

  This guaranty is [xx] unsecured; [ ] secured by a mortgage or security
agreement dated ___________; [ ] secured by ____________________________.

  IN WITNESS WHEREOF, this guaranty has been duly executed by the Undersigned
the day and year first above written.

                                      X
                                      -----------------------------------------
                                      MARK A. MCMILLAN
                                      -----------------------------------------
                                  
                                      -----------------------------------------
                                   
                                      -----------------------------------------
                                      "Undersigned" shall refer to all persons
                                      who sign this guaranty, severally and 
                                      jointly.

BANKERS SYSTEMS, INC., ST. CLOUD, MN 56301 (1-800-397-2341) FORM M-240 5/20/91
(For Corporate Guarantor use M-250)                                (page 1 of 2)

                              


<PAGE>   2
         EXHIBIT 10(a)(4) - SCHEDULE OF SIMILAR GUARANTY AGREEMENTS


  Lender:   . . . . . . . . . .     The First National Bank of Bryan
                                    2807 Texas Avenue
                                    PO Box 833
                                    Bryan, Texas 77805
  Borrower:   . . . . . . . . .     BodyBilt Seating, Inc.
                                    4455 Carter Creek Parkway
                                    Bryan, Texas 77802
  Date:   . . . . . . . . . . .     May 1, 1995
  Loan Type:  . . . . . . . . .     Revolving Line of Credit (#033-118-60)
  Amount:   . . . . . . . . . .     $1 million
  Expiration:   . . . . . . . .     April 30, 1996
  Guarantor:  . . . . . . . . .     Mark McMillan
  Security:   . . . . . . . . .     None
  Liability Limit:  . . . . . .     None
