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                                                                    EXHIBIT 99.4
 
                              TRENWICK GROUP INC.
 
                           Offer for all Outstanding
                      6.70% Senior Notes due April 1, 2003
                                in Exchange for
                 6.70% Senior Exchange Notes due April 1, 2003,
                        which have been registered under
                     the Securities Act of 1933, as amended
 
To: Brokers, Dealers, Commercial Banks,
    Trust Companies and Other Nominees:
 
     Trenwick Group Inc. (the "Issuer") is offering, upon and subject to the
terms and conditions set forth in the prospectus dated                , 1998
(the "Prospectus"), and the enclosed letter of transmittal (the "Letter of
Transmittal"), to exchange (the "Exchange Offer") its 6.70% Senior Exchange
Notes due April 1, 2003, which have been registered under the Securities Act of
1933, as amended, for its outstanding 6.70% Senior Notes due April 1, 2003 (the
"Old Notes"). The Exchange Offer is being made in order to satisfy certain
obligations of the Issuer contained in the registration rights agreement in
respect of the Old Notes, dated as of March 27, 1998, between the Issuer and the
initial purchaser referred to therein.
 
     We are requesting that you contact your clients for whom you hold Old Notes
regarding the Exchange Offer. For your information and for forwarding to your
clients for whom you hold Old Notes registered in your name or in the name of
your nominee, or who hold Old Notes registered in their own names, we are
enclosing the following documents:
 
          1.  Prospectus dated                  , 1998;
 
          2.  The Letter of Transmittal for your use and for the information of
     your clients;
 
          3.  A Notice of Guaranteed Delivery to be used to accept the Exchange
     Offer if certificates for Old Notes are not immediately available or time
     will not permit all required documents to reach the Exchange Agent prior to
     the Expiration Date (as defined below) or if the procedure for book-entry
     transfer cannot be completed on a timely basis;
 
          4.  A form of letter which may be sent to your clients for whose
     account you hold Old Notes registered in your name or the name of your
     nominee, with space provided for obtaining such clients' instructions with
     regard to the Exchange Offer;
 
          5.  Guidelines for Certification of Taxpayer Identification Number on
     Substitute Form W-9; and
 
          6.  Return envelopes addressed to The First National Bank of Chicago,
     the Exchange Agent for the Exchange Offer.
 
     YOUR PROMPT ACTION IS REQUESTED. THE EXCHANGE OFFER WILL EXPIRE AT 5:00
P.M., NEW YORK CITY TIME, ON                  , 1998 UNLESS EXTENDED BY THE
ISSUER (THE "EXPIRATION DATE"). OLD NOTES TENDERED PURSUANT TO THE EXCHANGE
OFFER MAY BE WITHDRAWN AT ANY TIME BEFORE THE EXPIRATION DATE.
 
     To participate in the Exchange Offer, a duly executed and properly
completed Letter of Transmittal (or facsimile thereof or Agent's Message in lieu
thereof), with any required signature guarantees and any other required
documents, should be sent to the Exchange Agent and certificates representing
the Old Notes, or a timely confirmation of a book-entry transfer of such Old
Notes, should be delivered to the Exchange Agent, all in accordance with the
instructions set forth in the Letter of Transmittal and the Prospectus.
 
     If a registered holder of Old Notes desires to tender, but such Old Notes
are not immediately available, or time will not permit such holder's Old Notes
or other required documents to reach the Exchange Agent before the Expiration
Date, or the procedure for book-entry transfer cannot be completed on a timely
basis, a tender may be effected by following the guaranteed delivery procedures
described in the Prospectus under "The Exchange Offer -- Guaranteed Delivery."
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     The Issuer will, upon request, reimburse brokers, dealers, commercial banks
and trust companies for reasonable and necessary costs and expenses incurred by
them in forwarding the Prospectus and the related documents to the beneficial
owners of Old Notes held by them as nominee or in a fiduciary capacity. The
Issuer will not make any payments to brokers, dealers, or others soliciting
acceptances of the Exchange Offer. The holders will not be obligated to pay or
cause to be paid all stock transfer taxes applicable to the exchange of Old
Notes pursuant to the Exchange Offer.
 
     Any inquiries you may have with respect to the Exchange Offer, or requests
for additional copies of the enclosed materials, should be directed to The First
National Bank of Chicago, the Exchange Agent for the Exchange Offer, at its
address and telephone number set forth on the front of the Letter of
Transmittal.
 
                                      Very truly yours,
 
                                      Trenwick Group Inc.
 
     NOTHING HEREIN OR IN THE ENCLOSED DOCUMENTS SHALL CONSTITUTE YOU OR ANY
PERSON AS AN AGENT OF THE ISSUER OR THE EXCHANGE AGENT, OR AUTHORIZE YOU OR ANY
OTHER PERSON TO USE ANY DOCUMENT OR MAKE ANY STATEMENTS ON BEHALF OF EITHER OF
THEM WITH RESPECT TO THE EXCHANGE OFFER, EXCEPT FOR STATEMENTS EXPRESSLY MADE IN
THE PROSPECTUS OR THE LETTER OF TRANSMITTAL.
 
Enclosures
 
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