EXHIBIT 99.4
 
                                ONBANCORP, INC.
                       9.25% ORIGINAL CAPITAL SECURITIES
                (LIQUIDATION AMOUNT $1,000 PER CAPITAL SECURITY)
                                IN EXCHANGE FOR
                      9.25% EXCHANGE CAPITAL SECURITIES OF
                             ONBANK CAPITAL TRUST I
 
To: Brokers, Dealers, Commercial Banks,
   Trust Companies and Other Nominees:
 
    ONBANCorp, Inc. Corporation (the "Company") and OnBank Capital Trust I (the
"Trust") are offering, upon and subject to the terms and conditions set forth in
a prospectus dated       , 1997 (the "Prospectus"), and the enclosed letter of
transmittal (the "Letter of Transmittal"), to exchange (the "Exchange Offer")
the Trust's 9.25% Series B Capital Securities (the "Exchange Capital
Securities") for its outstanding 9.25% Series A Capital Securities (the
"Original Capital Securities"). The Exchange Offer is being made in order to
satisfy certain obligations of the Company and the Trust contained in the
Registration Rights Agreement, dated February 4, 1997, among the Company, the
Trust and the initial purchasers referred to therein.
 
    We are requesting that you contact your clients for whom you hold Original
Capital Securities regarding the Exchange Offer. For your information and for
forwarding to your clients for whom you hold Original Capital Securities
registered in your name or in the name of your nominee, or who hold Original
Capital Securities registered in their own names, we are enclosing the following
documents:
 
        1.  Prospectus dated       , 1997;
 
        2.  The Letter of Transmittal for your use and for the information (or
    the use, where relevant) of your clients;
 
        3.  A Notice of Guaranteed Delivery to be used to accept the Exchange
    Offer if certificates for Original Capital Securities 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 Original Capital Securities 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 Bank of New York, the Exchange
    Agent for Original Capital Securities.
 
    Your prompt action is requested. The Exchange Offer will expire at 5:00
p.m., New York City time, on       , 1997, unless extended by the Company or the
Trust (the "Expiration Date"). The Original Capital Securities 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 an Agent's Message (as defined in
the Prospectus) in lieu thereof), with any required signature guarantees and any
other required documents, should be sent to the Exchange Agent

and certificates representing the Original Capital Securities should be
delivered to the Exchange Agent, all in accordance with the instructions set
forth in the Letter of Transmittal and the Prospectus.
 
    If holders of Original Capital Securities wish to tender, but it is
impracticable for them to forward their certificates for Original Capital
Securities prior to the expiration of the Exchange Offer or to comply with the
book-entry transfer procedures 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 Procedures."
 
    The Company 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 Original Capital Securities held by them as nominee or in a fiduciary
capacity. The Company will pay or cause to be paid all stock transfer taxes
applicable to the exchange of Original Capital Securities pursuant to the
Exchange Offer, except as set forth in Instruction 6 of the Letter of
Transmittal.
 
    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 Bank
of New York, the Exchange Agent for the Original Capital Securities, at its
address and telephone number set forth on the front of the Letter of
Transmittal.
 
                                          Very truly yours,
 
                                          ONBANCorp, Inc.
                                          OnBank Capital Trust I
 
    NOTHING HEREIN OR IN THE ENCLOSED DOCUMENTS SHALL CONSTITUTE YOU OR ANY
PERSON AS AN AGENT OF THE COMPANY 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