1
                                                                       EXHIBIT 4


                                Eaton Corporation
                                  Eaton Center
                              Cleveland, Ohio 44114




                                  June 24, 1997


Fusion Systems Corporation
7600 Standish Place
Rockville, Maryland  20855

Gentlemen:

         To induce Eaton Corporation ("Eaton") to continue its due diligence
investigation of Fusion Systems Corporation (the "Company") and to pursue its
interest in a possible business combination with the Company, Eaton and the
Company have agreed as follows:

         (1) The Company agrees to negotiate exclusively with Eaton through July
7, 1997 or such later date as the parties may agree in writing (such period
being referred to herein as the "Exclusivity Period") with respect to a possible
business combination between the Company and Eaton.

         (2) During the Exclusivity Period, the Company agrees that it shall
not, and it shall not permit its subsidiaries, or any officers, directors,
employees, financial advisors and other agents or representatives of the Company
or its subsidiaries, directly or indirectly, to solicit or initiate (including
by way of furnishing any non-public information concerning the Company or its
assets) inquiries or proposals, or participate in any discussions or
negotiations with any person, corporation, partnership or other entity (other
than Eaton), concerning a merger or other business combination or an acquisition
of all or any substantial portion of the Company or its assets (a
"Transaction").

         (3) The Company and Eaton agree that they shall not, and shall cause
their officers, directors, agents and representatives not to, make any public
announcement or otherwise disclose to any person (other than the respective
officers, directors, employees, lawyers and financial advisors of Eaton and the
Company as required to negotiate a definitive agreement who are informed of the
confidential nature of this information) the fact that discussions or
negotiations are taking place concerning a possible transaction between the
Company and Eaton or any of the terms, conditions or other facts with respect to
any such possible transaction, including the status thereof unless the parties
agree to such disclosure or unless otherwise required by law.

   2

                                      -2-



         Please confirm your agreement with the foregoing by signing the
enclosed copy of this letter and returning it to us, whereupon it will become a
binding obligation of Eaton and the Company. The agreements set forth in this
letter may be modified or waived only by a separate writing by the Company and
Eaton expressly so modifying or waiving such agreements. Nothing herein
contained shall modify or supersede the obligations of the Company and Eaton
under the confidentiality Agreement dated April 7, 1997, which agreement remains
in full force and effect for the term provided therein.

                                                  Very truly yours,

                                                  EATON CORPORATION


                                                  By: /s/ Alexander M. Cutler
                                                     -------------------------


ACCEPTED AND AGREED:

FUSION SYSTEMS CORPORATION


By: /s/ Leslie S. Levine
   ------------------------

Date: June 24, 1997