<PAGE>   1


                                                                    EXHIBIT 4(c)






                          FIRST SUPPLEMENTAL INDENTURE

                                     between

                        FLEETBOSTON FINANCIAL CORPORATION

                                       and

                              THE BANK OF NEW YORK

                            Dated as of June 30, 2000




<PAGE>   2



                               Table of Contents*

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                                                    ARTICLE I
                                                   DEFINITIONS

SECTION 1.1   Definition of Terms.......................................................................2

                                                   ARTICLE II
                                  GENERAL TERMS AND CONDITIONS OF THE DEBENTURES

SECTION 2.1   Designation and Principal Amount..........................................................4
SECTION 2.2   Maturity..................................................................................4
SECTION 2.3   Form and Payment..........................................................................4
SECTION 2.4   Global Debenture..........................................................................4
SECTION 2.5   Interest..................................................................................5
SECTION 2.6   Denomination..............................................................................6

                                                  ARTICLE III
                                          REDEMPTION OF THE DEBENTURES

SECTION 3.1   Optional Redemption.......................................................................6
SECTION 3.2   Redemption Procedures.....................................................................7
SECTION 3.3   No Sinking Fund...........................................................................7
SECTION 3.4   Required Approval.........................................................................7

                                                  ARTICLE IV
                                   EXTENSION OF INTEREST PAYMENT PERIOD

SECTION 4.1   Extension of Interest Payment Period......................................................7
SECTION 4.2   Notice of Extension.......................................................................8
SECTION 4.3   Limitation of Transactions................................................................8

                                                  ARTICLE V
                                                  EXPENSES

SECTION 5.1   Payment of Expenses.......................................................................9
SECTION 5.2   Payment Upon Resignation or Removal......................................................10

                                                ARTICLE VI
                                       COVENANT TO LIST ON EXCHANGE

SECTION 6.1   Listing on an Exchange...................................................................10
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                                                ARTICLE VII
                                             FORM OF DEBENTURE

SECTION 7.1   Form of Debenture........................................................................10

                                               ARTICLE VIII
                                      ORIGINAL ISSUE OF DEBENTURES

SECTION 8.1   Original Issue of Debentures.............................................................16

                                                ARTICLE IX
                                              MISCELLANEOUS

SECTION 9.1   Ratification of Indenture................................................................16
SECTION 9.2   Trustee Not Responsible for Recitals.....................................................16
SECTION 9.3   Governing Law............................................................................16
SECTION 9.4   Separability.............................................................................17
SECTION 9.5   Counterparts.............................................................................17

* THIS TABLE OF CONTENTS SHALL NOT, FOR ANY PURPOSE, BE DEEMED TO BE A PART OF
  THIS FIRST SUPPLEMENTAL INDENTURE.
</TABLE>

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     FIRST SUPPLEMENTAL INDENTURE, dated as of June 30, 2000 (the "First
Supplemental Indenture"), between FleetBoston Financial Corporation, a Rhode
Island corporation (the "Company"), and The Bank of New York, a New York banking
corporation, as trustee (the "Trustee") under the Indenture dated as of June 30,
2000 between the Company and the Trustee (the "Indenture").

     WHEREAS, the Company executed and delivered the Indenture to the Trustee to
provide for the future issuance of the Company's unsecured junior subordinated
debt securities to be issued from time to time in one or more series as might be
determined by the Company under the Indenture, in an unlimited aggregate
principal amount which may be authenticated and delivered as provided in the
Indenture;

     WHEREAS, pursuant to the terms of the Indenture, the Company desires to
provide for the establishment of a new series of such securities to be known as
its 8.80% Junior Subordinated Deferrable Interest Debentures due 2030 (the
"Debentures"), the form and substance of such Debentures and the terms,
provisions and conditions thereof to be set forth as provided in the Indenture
and this First Supplemental Indenture;

     WHEREAS, the Company and Fleet Capital Trust VI, a Delaware statutory
business trust (the "Trust"), has offered to the public up to $316,250,000
aggregate liquidation amount of its 8.80% Trust Originated Preferred
Securities(SM) ("TOPrS(SM)") (the "Preferred Securities"), representing
preferred undivided beneficial interests in the assets of the Trust, and
proposes to invest the proceeds from such offering, together with the proceeds
of the issuance and sale by the Trust to the Company of up to $9,781,000
aggregate liquidation amount of its 8.80% Common Securities (the "Common
Securities"), in up to $326,031,000 aggregate principal amount of the
Debentures; and

     WHEREAS, the Company has requested that the Trustee execute and deliver
this First Supplemental Indenture and all requirements necessary to make this
First Supplemental Indenture a valid instrument in accordance with its terms,
and to make the Debentures, when executed by the Company and authenticated and
delivered by the Trustee, the valid obligations of the Company, have been
performed, and the execution and delivery of this First Supplemental Indenture
has been duly authorized in all respects.

     NOW THEREFORE, in consideration of the purchase and acceptance of the
Debentures by the Holders thereof, and for the purpose of setting forth, as
provided in the Indenture, the form and substance of the Debentures and the
terms, provisions and conditions thereof, the Company covenants and agrees with
the Trustee as follows:

----------------------
     SM "Trust Originated Preferred Securities and "TOPrS" are service marks of
Merrill Lynch & Co., Inc.


                                       1

<PAGE>   5




                                    ARTICLE I
                                   DEFINITIONS

     SECTION 1.1 Definition of Terms.

     Unless the context otherwise requires:

     (a) a term defined in the Indenture has the same meaning when used in this
First Supplemental Indenture;

     (b) a term defined anywhere in this First Supplemental Indenture has the
same meaning throughout;

     (c) the singular includes the plural and vice versa;

     (d) a reference to a Section or Article is to a Section or Article of this
First Supplemental Indenture;

     (e) headings are for convenience of reference only and do not affect
interpretation;

     (f) the following terms have the meanings given to them in the Declaration:
(i) Purchase Agreement; (ii) Delaware Trustee; (iii) Distributions; (iv)
Institutional Trustee; (v) Preferred Securities Guarantee; (vi) Preferred
Security Certificate; and (vii) Regular Trustee; and

     (g) the following terms have the meanings given to them in this Section
1.1(g):

     "Additional Interest" shall have the meaning set forth in Section 2.5(c).

     "Compound Interest" shall have the meaning set forth in Section 4.1.

     "Coupon Rate" shall have the meaning set forth in Section 2.5(a).

     "Creditor" shall have the meaning set forth in Section 5.1.

     "Declaration" means the Amended and Restated Declaration of Trust of Fleet
Capital Trust VI, a Delaware statutory business trust, dated as of June 30,
2000.

     "Deferred Interest" shall have the meaning set forth in Section 4.1.

     "Dissolution Event" means the dissolution of the Trust and distribution of
the Debentures held by the Institutional Trustee pro rata to the holders of the
Trust Securities in accordance with the Declaration, such event to occur at the
option of the Company at any time.

     "Extended Interest Payment Period" shall have the meaning set forth in
Section 4.1.

     "Federal Reserve Board" means the Board of Governors of the Federal Reserve
System.

                                      2

<PAGE>   6

     "Global Debenture" shall have the meaning set forth in Section 2.4(a).

     "Holder" means any person in whose name at the time a Debenture is
registered on the Security Register.

     "Interest Payment Date" shall have the meaning set forth in Section 2.5(a).

     "Non Book-Entry Preferred Securities" shall have the meaning set forth in
Section 2.4(a).

     "Prepayment Price" shall have the meaning set forth in Section 3.1.

     "Regulatory Capital Event" means that the Company shall have received an
opinion of independent bank regulatory counsel experienced in such matters to
the effect that, as a result of (a) any amendment to, or change (including any
announced prospective change) in, the laws (or any regulations thereunder) of
the United States or any rules, guidelines or policies of the Federal Reserve
Board or (b) any official administrative pronouncement or judicial decision
interpreting or applying such laws or regulations, which amendment or change is
effective or such pronouncement or decision is announced on or after the date of
original issuance of the Preferred Securities, the Preferred Securities do not
constitute, or within 90 days of the date thereof, will not constitute, Tier 1
capital (or its equivalent) for purposes of the Federal Reserve Board's capital
guidelines for bank holding companies; provided, however, that the distribution
of the Debentures in connection with the liquidation of the Trust by the Company
and the treatment thereafter of the Debentures as other than Tier 1 capital
shall not in and of itself constitute a Regulatory Capital Event unless such
liquidation shall have occurred in connection with a Tax Event.

     "Special Event" means a Tax Event or a Regulatory Capital Event, as the
case may be.

     "Stated Maturity" means the date on which the Debentures mature and on
which the principal shall be due and payable, together with all accrued and
unpaid interest thereon including Compound Interest and Additional Interest, if
any, which date shall be June 30, 2030.

     "Tax Event" means that the Regular Trustees shall have received an opinion
of a nationally recognized independent tax counsel experienced in such matters
to the effect that, as a result of (a) any amendment to, or change (including
any announced prospective change) in, the laws or any regulations thereunder of
the United States or any political subdivision or taxing authority thereof or
therein, or (b) any official administrative pronouncement or judicial decision
interpreting or applying such laws or regulations, which amendment or change is
effective or such pronouncement or decision is announced on or after the date of
the original issuance of the Debentures, there is more than an insubstantial
risk that (i) the Trust is, or will be within 90 days of the date of such
opinion, subject to United States federal income tax with respect to income
received or accrued on the Debentures, (ii) interest payable by the Company on
the Debentures is not, or within 90 days of the date thereof will not be,
deductible by the Company, in whole or in part, for United States federal income
tax purposes, or (iii) the Trust is, or will be within 90 days of the date of
such opinion, subject to more than a de minimis amount of other taxes, duties or
other governmental charges.

                                       3

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                                   ARTICLE II
                 GENERAL TERMS AND CONDITIONS OF THE DEBENTURES

     SECTION 2.1 Designation and Principal Amount.

     There is hereby authorized a series of Securities designated as the "8.80%
Junior Subordinated Deferrable Interest Debentures due 2030", in aggregate
principal amount of up to $326,031,000 which amount shall be as set forth in any
written order of the Company for the authentication and delivery of Debentures
pursuant to Section 2.04 of the Indenture.

     SECTION 2.2 Maturity.

     The Debentures shall mature on June 30, 2030 (the "Stated Maturity").

     SECTION 2.3 Form and Payment.

     Except as provided in Section 2.4, the Debentures shall be issued in fully
registered certificated form without interest coupons. Principal and interest on
the Debentures issued in certificated form will be payable, the transfer of such
Debentures will be registrable and such Debentures will be exchangeable for
Debentures bearing identical terms and provisions at the office or agency of the
Trustee in New York, New York; provided, however, that payment of interest may
be made at the option of the Company by check mailed to the Holder entitled
thereto at such address as shall appear in the Security Register or by wire
transfer to an account appropriately designated by the Holder, entitled thereto.
Notwithstanding the foregoing, so long as the Holder of any Debentures is the
Institutional Trustee, the payment of the principal of and interest (including
Compound Interest and Additional Interest, if any) on such Debentures held by
the Institutional Trustee will be made at such place and to such account as may
be designated by the Institutional Trustee.

     SECTION 2.4 Global Debenture.

     (a) In connection with a Dissolution Event,

          (i) the Debentures in certificated form may be presented to the
     Trustee by the Institutional Trustee in exchange for a global Debenture in
     an aggregate principal amount equal to the aggregate principal amount of
     all outstanding Debentures (a "Global Debenture"), to be registered in the
     name of the Depository Institution, or its nominee, and delivered by the
     Trustee to the Depository Institution for crediting to the accounts of its
     participants pursuant to the instructions of the Regular Trustees. The
     Company upon any such presentation shall execute a Global Debenture in such
     aggregate principal amount and deliver the same to the Trustee for
     authentication and delivery in accordance with the Indenture and this First
     Supplemental Indenture. Payments on the Debentures issued as a Global
     Debenture will be made to the Depository Institution; and

          (ii) if any Preferred Securities are held in non book-entry
     certificated form, the Debentures in certificated form may be presented to
     the Trustee by the Institutional Trustee and any Preferred Security
     Certificate which represents Preferred Securities other than Preferred
     Securities held by the Depository Institution or its nominee ("Non
     Book-

                                       4
<PAGE>   8


     Entry Preferred Securities") will be deemed to represent beneficial
     interests in Debentures presented to the Trustee by the Institutional
     Trustee having an aggregate principal amount equal to the aggregate
     liquidation amount of the Non Book-Entry Preferred Securities until such
     Preferred Security Certificates are presented to the Security registrar for
     transfer or reissuance, at which time such Preferred Security Certificates
     will be canceled and a Debenture, registered in the name of the holder of
     the Preferred Security Certificate or the transferee of the holder of such
     Preferred Security Certificate, as the case may be, with an aggregate
     principal amount equal to the aggregate liquidation amount of the Preferred
     Security Certificate canceled, will be executed by the Company and
     delivered to the Trustee for authentication and delivery in accordance with
     the Indenture and this First Supplemental Indenture. On issue of such
     Debentures, Debentures with an equivalent aggregate principal amount that
     were presented by the Institutional Trustee to the Trustee will be deemed
     to have been canceled.

          (b) A Global Debenture may be transferred, in whole but not in part,
     only to another nominee of the Depository Institution, or to a successor
     Depository Institution selected or approved by the Company or to a nominee
     of such successor Depository Institution.

          (c) If (i) at any time the Depository Institution notifies the Company
     that it is unwilling or unable to continue as Depository Institution or if
     at any time the Depository Institution for such series shall no longer be
     registered or in good standing under the Securities Exchange Act of 1934,
     as amended, or other applicable statute or regulation, and a successor
     Depository Institution for such series is not appointed by the Company
     within 90 days after the Company receives such notice or becomes aware of
     such condition, as the case may be, (ii) the Company at any time determines
     that the Debentures shall no longer be solely represented by a Global
     Debenture or (iii) there shall have occurred an Event of Default, then the
     Company shall execute, and, subject to Article II of the Indenture, the
     Trustee, upon written notice from the Company, shall authenticate and
     deliver the Debentures in definitive registered form without coupons, in
     authorized denominations, and in an aggregate principal amount equal to the
     principal amount of the Global Debenture in exchange for such Global
     Debenture. In such event the Company shall execute, and, subject to Section
     2.07 of the Indenture, the Trustee, upon receipt of an Officers'
     Certificate evidencing such determination by the Company, shall
     authenticate and deliver the Debentures in definitive registered form
     without coupons, in authorized denominations, and in an aggregate principal
     amount equal to the principal amount of the Global Debenture in exchange
     for such Global Debenture. Upon the exchange of the Global Debenture for
     such Debentures in definitive registered form without coupons, in
     authorized denominations, the Global Debenture shall be canceled by the
     Trustee. Such Debentures in definitive registered form issued in exchange
     for the Global Debenture shall be registered in such names and in such
     authorized denominations as the Depository Institution, pursuant to
     instructions from its direct or indirect participants or otherwise, shall
     instruct the Trustee. The Trustee shall deliver such Securities to the
     Depository Institution for delivery to the Persons in whose names such
     Securities are so registered.

          SECTION 2.5 Interest.

     (a) Each Debenture will bear interest at the rate of 8.80% per annum (the
"Coupon Rate") from June 30, 2000 until the principal thereof becomes due and
payable, and on any

                                       5

<PAGE>   9

overdue principal and, to the extent that payment of such interest is
enforceable under applicable law, on any overdue installment of interest at the
Coupon Rate, compounded quarterly, payable quarterly in arrears on March 31,
June 30, September 30 and December 31 of each year (each, an "Interest Payment
Date"), commencing on September 30, 2000, to the Person in whose name such
Debenture or any predecessor Debenture is registered at the close of business on
the relevant record date, which will be, as long as the Preferred Securities
remain in book-entry form (or if no Preferred Securities remain outstanding, as
long as the Debentures remain in book entry form), one Business Day prior to the
relevant Interest Payment Date and, in the event the Preferred Securities are
not in book-entry form (or if no Preferred Securities remain outstanding, in the
event the Debentures are not in book entry form), the 15th day of the month in
which the relevant Interest Payment Date date occurs, except as otherwise
provided pursuant to the provisions of Article IV hereof.

     (b) The amount of interest payable for any period will be computed on the
basis of a 360-day year of twelve 30-day months. Except as provided in the
following sentence, the amount of interest payable for any period shorter than a
full quarterly period for which interest is computed, will be computed on the
basis of the actual number of days elapsed per calendar month (but not to exceed
30 days in any month). In the event that any date on which interest is payable
on the Debentures is not a Business Day, then payment of interest payable on
such date will be made on the next succeeding day which is a Business Day (and
without any interest or other payment in respect of any such delay), except
that, if such Business Day is in the next succeeding calendar year, such payment
shall be made on the immediately preceding Business Day, in each case with the
same force and effect as if made on the date that such interest otherwise would
have been payable.

     (c) If, at any time while the Institutional Trustee is the holder of any
Junior Subordinated Debentures, the Trust or the Institutional Trustee is
required to pay any taxes, duties, assessments or governmental charges of
whatever nature (other than withholding taxes) imposed by the United States, or
any other taxing authority, then, in any such case, the Company will pay as
additional interest ("Additional Interest") on the Debentures held by the
Institutional Trustee, such additional amounts as shall be required so that the
net amounts received and retained by the Trust and by the Institutional Trustee
after paying such taxes, duties, assessments or other governmental charges will
be equal to the amounts the Trust and the Institutional Trustee would have
received had no such taxes, duties, assessments or other governmental charges
been imposed.

     SECTION 2.6 Denomination.

     The Debentures shall be issued in denominations of $25 and integral
multiples thereof.

                                  ARTICLE III
                          REDEMPTION OF THE DEBENTURES

     SECTION 3.1 Optional Redemption.

     The Junior Subordinated Debentures are prepayable prior to the Stated
Maturity at the option of the Company (i) in whole or in part, from time to
time, on or after June 30, 2005

                                       6
<PAGE>   10

or (ii) at any time prior to June 30, 2005, in whole but not in part, upon the
occurrence and continuation of a Special Event, in either case at a prepayment
price (the "Prepayment Price") equal to 100% of the principal amount thereof,
plus accrued and unpaid interest thereon (including Additional Interest and
Compound Interest, if any) to the date of prepayment.

     SECTION 3.2 Redemption Procedures.

     Notice of any redemption will be mailed at least 30 days but not more than
60 days before the redemption date to each Holder of Debentures to be prepaid at
its registered address. Unless the Company defaults in payment of the prepayment
price, on and after the redemption date interest shall cease to accrue on such
Debentures called for redemption. If the Debentures are only partially redeemed
pursuant to Section 3.1, the Debentures will be redeemed pro rata or by lot or
by any other method utilized by the Trustee; provided, that if at the time of
redemption the Debentures are registered as a Global Debenture, the Depository
Institution shall determine, in accordance with its procedures, the principal
amount of such Debentures held by each Depository Institution participant to be
redeemed. The Prepayment Price shall be paid prior to 12:00 noon, New York time,
on the date of such prepayment or at such earlier time as the Company
determines; provided that the Company shall deposit with the Trustee an amount
sufficient to pay the Prepayment Price by 10:00 a.m., New York time, on the date
such prepayment price is to be paid.

     SECTION 3.3 No Sinking Fund.

     The Debentures are not entitled to the benefit of any sinking fund.

     SECTION 3.4 Required Approval.

     Any redemption of the Debentures in accordance with the foregoing Sections
may require the prior approval of the Federal Reserve Board if such approval is
then required under applicable law, rules, guidelines or policies.

                                   ARTICLE IV
                      EXTENSION OF INTEREST PAYMENT PERIOD

     SECTION 4.1 Extension of Interest Payment Period.

     So long as the Company shall not be in default in the payment of interest
on the Debentures, the Company shall have the right, at any time and from time
to time during the term of the Debentures, to defer payments of interest by
extending the interest payment period of such Debentures for a period not
exceeding 20 consecutive quarters (the "Extended Interest Payment Period"),
during which Extended Interest Payment Period no interest shall be due and
payable; provided that no Extended Interest Payment Period may extend beyond the
Stated Maturity. To the extent permitted by applicable law, interest, the
payment of which has been deferred because of the extension of the interest
payment period pursuant to this Section 4.1, will bear interest thereon at the
Coupon Rate compounded quarterly for each quarter of the Extended Interest
Payment Period ("Compound Interest"). At the end of the Extended Interest
Payment Period, the Company shall pay all interest accrued and unpaid on the
Debentures, including any Additional Interest and Compound Interest (together,
"Deferred Interest") that shall be payable to the

                                       7
<PAGE>   11

Holders in whose names the Debentures are registered in the Security Register on
the record date for the first Interest Payment Date after the end of the
Extended Interest Payment Period. Before the termination of any Extended
Interest Payment Period, the Company may further extend such period, provided
that such period together with all such further extensions thereof shall not
exceed 20 consecutive quarters, or extend beyond the Stated Maturity of the
Debentures. Upon the termination of any Extended Interest Payment Period and
upon the payment of all Deferred Interest then due, the Company may commence a
new Extended Interest Payment Period, subject to the foregoing requirements. No
interest shall be due and payable during an Extended Interest Payment Period,
except at the end thereof, but the Company may prepay at any time all or any
portion of the interest accrued during an Extended Interest Payment Period.

     SECTION 4.2 Notice of Extension.

     (a) If the Institutional Trustee is the only registered Holder at the time
the Company selects an Extended Interest Payment Period, the Company shall give
written notice to the Regular Trustees, the Institutional Trustee and the
Trustee of its selection of such Extended Interest Payment Period one Business
Day before the earlier of (i) the next succeeding date on which Distributions on
the Trust Securities issued by the Trust are payable, or (ii) the date the Trust
is required to give notice of the record date, or the date such Distributions
are payable, to the New York Stock Exchange or other applicable self-regulatory
organization or to holders of the Preferred Securities issued by the Trust, but
in any event at least one Business Day before such record date.

     (b) If the Institutional Trustee is not the only Holder at the time the
Company selects an Extended Interest Payment Period, the Company shall give the
Holders of the Debentures and the Trustee written notice of its selection of
such Extended Interest Payment Period at least ten Business Days before the
earlier of (i) the next succeeding Interest Payment Date, or (ii) the date the
Company is required to give notice of the record date or the Interest Payment
Date to the New York Stock Exchange or other applicable self-regulatory
organization or to Holders of the Debentures.

     (c) The quarter in which any notice is given pursuant to paragraphs (a) or
(b) of this Section 4.2 shall be counted as one of the 20 quarters permitted in
the maximum Extended Interest Payment Period permitted under Section 4.1.

     SECTION 4.3 Limitation of Transactions.

     If (i) the Company shall exercise its right to defer payment of interest as
provided in Section 4.1 and the Extended Interest Payment Period is continuing,
or (ii) there shall have occurred any Event of Default, as defined in the
Indenture, or (iii) there shall have occurred any Event of Default, as defined
in the Preferred Securities Guarantee, then (a) the Company shall not declare or
pay any dividend on, make any distribution with respect to, or redeem, purchase,
acquire or make a liquidation payment with respect to, any of its capital stock
(other than (1) purchases or acquisitions of shares of its common stock in
connection with the satisfaction by the Company of its obligations under any
employee benefit plans or any other contractual obligation of the Company (other
than a contractual obligation ranking pari passu with or junior to the
Debentures), (2) as a result of a reclassification of the Company's capital
stock or the

                                       8

<PAGE>   12

exchange or conversion of one class or series of the Company's capital stock for
another class or series of the Company's capital stock or (3) the purchase of
fractional interests in shares of the Company's capital stock pursuant to the
conversion or exchange provisions of such capital stock or the security being
converted or exchanged), (b) the Company shall not make any payment of interest,
principal or premium, if any, on or repay, repurchase or redeem any debt
securities issued by the Company that rank pari passu with or junior to the
Debentures and (c) the Company shall not make any guarantee payments with
respect to the foregoing (other than pursuant to the Preferred Securities
Guarantee).

                                   ARTICLE V
                                    EXPENSES

     SECTION 5.1 Payment of Expenses.

     In connection with the offering, sale and issuance of the Debentures to the
Institutional Trustee and in connection with the sale of the Trust Securities by
the Trust, the Company, in its capacity as borrower with respect to the
Debentures, shall:

     (a) pay all costs and expenses relating to the offering, sale and issuance
of the Debentures, including commissions to the underwriter payable pursuant to
the Purchase Agreement and compensation of the Trustee under the Indenture in
accordance with the provisions of Section 6.06 of the Indenture;

     (b) be responsible for and shall pay all debts and obligations (other than
with respect to the Trust Securities) and all costs and expenses of the Trust
(including, but not limited to, costs and expenses relating to the organization,
maintenance and dissolution of the Trust, the offering, sale and issuance of the
Trust Securities (including commissions to the underwriters in connection
therewith), the fees and expenses (including reasonable counsel fees and
expenses) of the Institutional Trustee, the Delaware Trustee and the Regular
Trustees (including any amounts payable under Article 10 of the Declaration),
the costs and expenses relating to the operation of the Trust, including without
limitation, costs and expenses of accountants, attorneys, statistical or
bookkeeping services, expenses for printing and engraving and computing or
accounting equipment, paying agent(s), registrar(s), transfer agent(s),
duplicating, travel and telephone and other telecommunications expenses and
costs and expenses incurred in connection with the acquisition, financing, and
disposition of Trust assets and the enforcement by the Institutional Trustee of
the rights of the holders of the Preferred Securities);

     (c) be liable for any indemnification obligations arising with respect to
the Declaration; and

     (d) pay any and all taxes (other than United States withholding taxes
attributable to the Trust or its assets) and all liabilities, costs and expenses
with respect to such taxes of the Trust.

     The Company's obligations under this Section 5.1 shall be for the benefit
of, and shall be enforceable by, any Person to whom such debts, obligations,
costs, expenses and taxes are owed (a "Creditor") whether or not such Creditor
has received notice hereof. Any such Creditor may enforce the Company's
obligations under this Section 5.1 directly against the Company and the

                                       9

<PAGE>   13

Company irrevocably waives any right of remedy to require that any such Creditor
take any action against the Trust or any other Person before proceeding against
the Company. The Company agrees to execute such additional agreements as may be
necessary or desirable in order to give full effect to the provisions of this
Section 5.1.

     The provisions of this Section shall survive the termination of this
Supplemental Indenture.

     SECTION 5.2 Payment Upon Resignation or Removal.

     Upon termination of this First Supplemental Indenture or the Indenture or
the removal or resignation of the Trustee, unless otherwise stated, the Company
shall pay to the Trustee all amounts accrued to the date of such termination,
removal or resignation that are payable pursuant to Section 6.06 of the
Indenture. Upon termination of the Declaration or the removal or resignation of
the Delaware Trustee or the Institutional Trustee, as the case may be, pursuant
to Section 5.6 of the Declaration, the Company shall pay to the Delaware Trustee
or the Institutional Trustee, as the case may be, all amounts accrued to the
date of such termination, removal or resignation.

                                   ARTICLE VI
                          COVENANT TO LIST ON EXCHANGE

     SECTION 6.1 Listing on an Exchange.

     If the Debentures are distributed to the holders of the Securities issued
by the Trust, and the Preferred Securities are then so listed, the Company will
use its best efforts to list such Debentures on the New York Stock Exchange,
Inc. or on such other exchange as the Preferred Securities are then listed.

                                  ARTICLE VII
                                FORM OF DEBENTURE

     SECTION 7.1 Form of Debenture.

     The Debentures and the Trustee's Certificate of Authentication to be
endorsed thereon are to be substantially in the following forms:

                           (FORM OF FACE OF DEBENTURE)

     IF THE DEBENTURE IS TO BE A GLOBAL DEBENTURE, INSERT - This Debenture is a
Global Debenture within the meaning of the Indenture hereinafter referred to and
is registered in the name of a Depositary or a nominee of a Depositary. This
Debenture is exchangeable for Debentures registered in the name of a person
other than the Depositary or its nominee only in the limited circumstances
described in the Indenture, and no transfer of this Debenture (other than a
transfer of this Debenture as a whole by the Depositary to a nominee of the
Depositary or by a nominee of the Depositary to the Depositary or another
nominee of the Depositary) may be registered except in limited circumstances.

                                       10

<PAGE>   14


     Unless this Debenture is presented by an authorized representative of The
Depository Trust Company (55 Water Street, New York, New York) to the issuer or
its agent for registration of transfer, exchange or payment, and any Debenture
issued is registered in the name of Cede & Co. or such other name as requested
by an authorized representative of The Depository Trust Company and any payment
hereon is made to Cede & Co., ANY TRANSFER, PLEDGE OR OTHER USE HEREOF FOR VALUE
OR OTHERWISE BY A PERSON IS WRONGFUL since the registered owner hereof, Cede &
Co., has an interest herein.

                                      No. 1
                        FLEETBOSTON FINANCIAL CORPORATION
             8.80% JUNIOR SUBORDINATED DEFERRABLE INTEREST DEBENTURE
                                    DUE 2030
$                                                               CUSIP#__________

     FLEETBOSTON FINANCIAL CORPORATION, a Rhode Island corporation (the
"Company", which term includes any successor corporation under the Indenture
hereinafter referred to), for value received, hereby promises to pay to The Bank
of New York, as Institutional Trustee of Fleet Capital Trust VI under that
certain Amended and Restated Declaration of Trust dated as of June 30, 2000, or
registered assigns, the principal sum of [ ] ($__________) on June 30, 2030,
(the "Stated Maturity"), and to pay interest on said principal sum from June 30,
2000, or from the most recent interest payment date (each such date, an
"Interest Payment Date") to which interest has been paid or duly provided for,
quarterly (subject to deferral as set forth herein) in arrears on March 31, June
30, September 30 and December 31 of each year commencing September 30, 2000, at
the rate of 8.80% per annum (the "Coupon Rate") until the principal hereof shall
have become due and payable, and on any overdue principal and premium, and
(without duplication and to the extent that payment of such interest is
enforceable under applicable law) on any overdue installment of interest at the
same rate per annum compounded quarterly. The amount of interest payable on any
Interest Payment Date shall be computed on the basis of a 360-day year of twelve
30-day months and, except as provided in the following sentences, the amount of
interest payable for any period shorter than a full quarterly period for which
interest is computed, will be computed on the basis of the actual number of days
elapsed per calendar month (but not to exceed 30 days in any month). In the
event that any date on which interest is payable on this Debenture is not a
Business Day, then payment of interest payable on such date will be made on the
next succeeding day that is a Business Day (and without any interest or other
payment in respect of any such delay), except that, if such Business Day is in
the next succeeding calendar year, such payment shall be made on the immediately
preceding Business Day, in each case with the same force and effect as if made
on the date such payment otherwise would have been payable. The interest
installment so payable, and punctually paid or duly provided for, on any
Interest Payment Date will, as provided in the Indenture, be paid to the person
in whose name this Debenture (or one or more Predecessor Securities, as defined
in said Indenture) is registered at the close of business on the relevant record
dates, which will be, as long as this Debenture remains in book-entry form, one
Business Day prior to the relevant Interest Payment Date and, in the event this
Debenture is not in book-entry form, the 15th day of the month in which the
relevant Interest Payment Date occurs. Payments of interest may be deferred by
the Company pursuant to the provisions of Article IV of the First Supplemental
Indenture to the Indenture (as defined herein). Any such interest installment
not punctually paid or duly provided for shall forthwith cease to be payable

                                       11

<PAGE>   15


to the registered Holders on such regular record date and may be paid to the
Person in whose name this Debenture (or one or more Predecessor Securities) is
registered at the close of business on a special record date to be fixed by the
Trustee for the payment of such defaulted interest, notice whereof shall be
given to the registered Holders of this series of Debentures not less than 10
days prior to such special record date, or may be paid at any time in any other
lawful manner not inconsistent with the requirements of any securities exchange
on which the Debentures may be listed, and upon such notice as may be required
by such exchange, all as more fully provided in the Indenture. The principal of
(and premium, if any) and the interest on this Debenture shall be payable at the
office or agency of the Trustee maintained for that purpose in any coin or
currency of the United States of America that at the time of payment is legal
tender for payment of public and private debts; provided, however, that payment
of interest may be made at the option of the Company by check mailed to the
registered Holder at such address as shall appear in the Security Register.
Notwithstanding the foregoing, so long as the Holder of this Debenture is the
Institutional Trustee, the payment of the principal of (and premium, if any) and
interest on this Debenture will be made at such place and to such account as may
be designated by the Institutional Trustee.

     The indebtedness evidenced by this Debenture is, to the extent provided in
the Indenture, subordinate and junior in right of payment to the prior payment
in full of all Senior Indebtedness and Other Financial Obligations (each as
defined in the Indenture) and this Debenture is issued subject to the provisions
of the Indenture with respect thereto. Each Holder of this Debenture, by
accepting the same, (a) agrees to and shall be bound by such provisions, (b)
authorizes and directs the Trustee on his or her behalf to take such action as
may be necessary or appropriate to acknowledge or effectuate the subordination
so provided and (c) appoints the Trustee his or her attorney-in-fact for any and
all such purposes. Each Holder hereof, by his or her acceptance hereof, hereby
waives all notice of the acceptance of the subordination provisions contained
herein and in the Indenture by each holder of Senior Indebtedness and Other
Financial Obligations, whether now outstanding or hereafter incurred, and waives
reliance by each such holder upon said provisions.

     This Debenture shall not be entitled to any benefit under the Indenture
hereinafter referred to, be valid or become obligatory for any purpose until the
Certificate of Authentication hereon shall have been signed by or on behalf of
the Trustee.

     THIS DEBENTURE IS NOT A SAVINGS OR DEPOSIT ACCOUNT OR OTHER OBLIGATION OF A
BANK AND IS NOT INSURED BY THE FEDERAL DEPOSIT INSURANCE CORPORATION OR ANY
OTHER GOVERNMENTAL AGENCY.

     The provisions of this Debenture are continued on the reverse side hereof
and such continued provisions shall for all purposes have the same effect as
though fully set forth at this place.

                                       12


<PAGE>   16


     IN WITNESS WHEREOF, the Company has caused this instrument to be executed.

                                     FLEETBOSTON FINANCIAL CORPORATION


                                     By:____________________________________
                                     Name:
                                     Title:
Attest:


By:______________________________________
Name:
Title:


                     (FORM OF CERTIFICATE OF AUTHENTICATION)
                          CERTIFICATE OF AUTHENTICATION


Dated: [    ]

     This is one of the Securities of the series designated herein referred to
in the within-mentioned Indenture.

The Bank of New York,
as Trustee

By:_______________________________________
             Authorized Signatory

                         (FORM OF REVERSE OF DEBENTURE)


     This Debenture is one of a duly authorized series of Debentures of the
Company (herein sometimes referred to as the "Debentures"), specified in the
Indenture, all issued or to be issued in one or more series under and pursuant
to an Indenture dated as of June 30, 2000, duly executed and delivered between
the Company and The Bank of New York, as trustee (the "Trustee"), as
supplemented by the First Supplemental Indenture dated as of June 30, 2000
between the Company and the Trustee (the Indenture as so supplemented, the
"Indenture"), to which Indenture and all indentures supplemental thereto
reference is hereby made for a description of the rights, limitations of rights,
obligations, duties and immunities thereunder of the Trustee, the Company and
the Holders of the Debentures. By the terms of the Indenture, the Debentures are
issuable in series that may vary as to amount, date of maturity, rate of
interest and in other respects as provided in the Indenture. This series of
Debentures is limited in aggregate principal amount as specified in said First
Supplemental Indenture.

                                     13

<PAGE>   17

     The Debenture is redeemable by the Company (i) in whole but not in part at
any time prior to June 30, 2005 upon the occurrence and continuation of a
Special Event (as defined in the Indenture) or (ii) in whole or in part on or
after June 30, 2005. Any redemption pursuant to this paragraph will be made upon
not less than 30 days nor more than 60 days notice, at a redemption price equal
to 100% of the principal amount plus any accrued but unpaid interest thereon
(including Additional Interest and Compound Interest, if any) to the date of
such redemption (the "Prepayment Price"). The Prepayment Price shall be paid
prior to 12:00 noon, New York time, on the date of such redemption or at such
earlier time as the Company determines. If the Debentures are only partially
redeemed by the Company pursuant to an Optional Redemption, the Debentures will
be redeemed pro rata or by lot or by any other method utilized by the Trustee
(in integral multiples of $25); provided that if, at the time of redemption, the
Debentures are registered as a Global Debenture, the Depositary shall determine
the principal amount of such Debentures held by each Debenture holder to be
redeemed in accordance with its procedures.

     In the event of redemption of this Debenture in part only, a new Debenture
or Debentures of this series for the unredeemed portion hereof will be issued in
the name of the Holder hereof upon the cancellation hereof.

     In case an Event of Default, as defined in the Indenture, shall have
occurred and be continuing, the principal of all of the Debentures may be
declared, and upon such declaration shall become, due and payable, in the
manner, with the effect and subject to the conditions provided in the Indenture.

     The Indenture contains provisions permitting the Company and the Trustee,
with the consent of the Holders of not less than a majority in aggregate
principal amount of the Debentures of each series affected at the time
outstanding, as defined in the Indenture, to execute supplemental indentures for
the purpose of adding any provisions to or changing in any manner or eliminating
any of the provisions of the Indenture or of any supplemental indenture or of
modifying in any manner the rights of the Holders of the Debentures; provided,
however, that no such supplemental indenture shall (i) extend the fixed maturity
of any Debentures of any series, or reduce the principal amount thereof or any
premium thereon, or reduce the rate or extend the time of payment of interest
thereon, or reduce any amount payable on redemption thereof or make the
principal thereon or any interest or premium thereon payable in any coin or
currency other than that provided in this Debenture, or impair or affect the
right of any Holder of a Debenture to institute suit for payment thereof or the
right of repayment, if any, at the option of the Holder, without the consent of
the Holder of each Debenture so affected, or (ii) reduce the aforesaid
percentage of Debentures, the Holders of which are required to consent to any
such supplemental indenture, without the consent of the Holders of each
Debenture then outstanding and affected thereby. The Indenture also contains
provisions permitting the Holders of a majority in aggregate principal amount of
the Debentures of any series at the time outstanding affected thereby, on behalf
of all of the Holders of the Debentures of such series, to waive any past
default in the performance of any of the covenants contained in the Indenture,
or established pursuant to the Indenture with respect to such series, and its
consequences, except a default in the payment of the principal of or premium, if
any, or interest on any of the Debentures of such series. Any such consent or
waiver by the registered Holder of this Debenture (unless revoked as provided in
the Indenture) shall be conclusive and binding upon such Holder and upon all
future Holders and owners of this Debenture and of any Debenture issued in
exchange hereof or in

                                       14

<PAGE>   18

place hereof (whether by registration of transfer or otherwise), irrespective of
whether or not any notation of such consent or waiver is made upon this
Debenture.

     No reference herein to the Indenture and no provision of this Debenture or
of the Indenture shall alter or impair the obligation of the Company, which is
absolute and unconditional, to pay the principal of and premium, if any, and
interest on this Debenture at the time and place and at the rate and in the
money herein prescribed.

     The Company shall have the right at any time during the term of the
Debentures and from time to time to extend the interest payment period of such
Debentures for up to 20 consecutive quarters (an "Extended Interest Payment
Period"), at the end of which period the Company shall pay all interest then
accrued and unpaid (together with interest thereon at the rate specified for the
Debentures to the extent that payment of such interest is enforceable under
applicable law); provided that no Extended Interest Payment Period may last
beyond the Stated Maturity of the Debentures. Before the termination of any such
Extended Interest Payment Period, the Company may further extend such Extended
Interest Payment Period, provided that such Extended Interest Payment Period
together with all such further extensions thereof shall not exceed 20
consecutive quarters or last beyond the Stated Maturity date of the Debentures.
At the termination of any such Extended Interest Payment Period and upon the
payment of all accrued and unpaid interest, including any Additional Interest
and Compound Interest, the Company may commence a new Extended Interest Payment
Period.

     As provided in the Indenture and subject to certain limitations therein set
forth, this Debenture is transferable by the registered Holder hereof on the
Security Register, upon surrender of this Debenture for registration of transfer
at the office or agency of the Trustee in the City and State of New York,
accompanied by a written instrument or instruments of transfer in form
satisfactory to the Company or the Trustee duly executed by the registered
Holder hereof or his attorney duly authorized in writing, and thereupon one or
more new Debentures of authorized denominations and for the same aggregate
principal amount and series will be issued to the designated transferee or
transferees. No service charge will be made for any such transfer, but the
Company may require payment of a sum sufficient to cover any tax or other
governmental charge payable in relation thereto.

     Prior to due presentment for registration of transfer of this Debenture,
the Company, the Trustee, any paying agent, any transfer agent and any security
registrar may deem and treat the registered holder hereof as the absolute owner
hereof (whether or not this Debenture shall be overdue and notwithstanding any
notice of ownership or writing hereon made by anyone other than a security
registrar) for the purpose of receiving payment of or on account of the
principal hereof and premium, if any, and interest due hereon and for all other
purposes, and none of the Company, the Trustee, any paying agent, any transfer
agent or any security registrar shall be affected by any notice to the contrary.

     No recourse shall be had for the payment of the principal of or the
interest on this Debenture, or for any claim based hereon, or otherwise in
respect hereof, or based on or in respect of the Indenture, against any
incorporator, stockholder, officer or director, past, present or future, as
such, of the Company or of any predecessor or successor corporation, whether by
virtue of any constitution, statute or rule of law, or by the enforcement of any
assessment or

                                       15


<PAGE>   19



penalty or otherwise, all such liability being, by the acceptance hereof and as
part of the consideration for the issuance hereof, expressly waived and
released.

     The Debentures of this series are issuable only in registered form without
coupons in denominations of $25 and any integral multiple thereof. As provided
in the Indenture and subject to certain limitations herein and therein set
forth, Debentures of this series so issued are exchangeable for a like aggregate
principal amount of Debentures of this series of a different authorized
denomination, as requested by the Holder surrendering the same.

     All terms used in this Debenture that are defined in the Indenture shall
have the meanings assigned to them in the Indenture.

     This Debenture shall be governed by the internal laws of the State of New
York, and for all purposes shall be construed in accordance with the laws of
said State.

                                  ARTICLE VIII
                          ORIGINAL ISSUE OF DEBENTURES

     SECTION 8.1 Original Issue of Debentures.

     Debentures in the aggregate principal amount of up to $326,031,000, may,
upon execution of this First Supplemental Indenture or upon any written order of
the Company setting forth the amount therefor, be executed by the Company and
delivered to the Trustee for authentication, and the Trustee shall thereupon
authenticate and deliver said Debentures to or upon the written order of the
Company, signed by its Chairman, its President, or any Vice President and its
Treasurer, its Secretary, any Assistant Treasurer, or any Assistant Secretary,
without any further action by the Company.

                                   ARTICLE IX
                                  MISCELLANEOUS

     SECTION 9.1 Ratification of Indenture.

     The Indenture, as supplemented by this First Supplemental Indenture, is in
all respects ratified and confirmed, and this First Supplemental Indenture shall
be deemed part of the Indenture in the manner and to the extent herein and
therein provided.

     SECTION 9.2 Trustee Not Responsible for Recitals.

     The recitals herein contained are made by the Company and not by the
Trustee, and the Trustee assumes no responsibility for the correctness thereof.
The Trustee makes no representation as to the validity or sufficiency of this
First Supplemental Indenture.

     SECTION 9.3 Governing Law.

     This First Supplemental Indenture and each Debenture shall be deemed to be
a contract made under the internal laws of the State of New York, and for all
purposes shall be construed in accordance with the laws of such State.

                                       16

<PAGE>   20



     SECTION 9.4 Separability.

     In case any one or more of the provisions contained in this First
Supplemental Indenture or in the Debentures shall for any reason be held to be
invalid, illegal or unenforceable in any respect, such invalidity, illegality or
unenforceability shall not affect any other provisions of this First
Supplemental Indenture or of the Debentures, but this First Supplemental
Indenture and the Debentures shall be construed as if such invalid or illegal or
unenforceable provision had never been contained herein or therein.

     SECTION 9.5 Counterparts.

     This First Supplemental Indenture may be executed in any number of
counterparts each of which shall be an original; but such counterparts shall
together constitute but one and the same instrument.


                                       17


<PAGE>   21


     IN WITNESS WHEREOF, the parties hereto have caused this First Supplemental
Indenture to be duly executed by their respective officers thereunto duly
authorized as of the day and year first above written.


[Seal]                                 FLEETBOSTON FINANCIAL CORPORATION

Attest:


By: /s/ JANICE B. LIVA                 By: /s/ DOUGLAS L. JACOBS
   -------------------------------         -----------------------------------
   Name: Janice B. Liva                    Name: Douglas L. Jacobs
   Title: Assistant Secretary              Title: Senior Vice President and
                                                  Treasurer

                                       THE BANK OF NEW YORK,
                                       as Trustee


                                       By: /s/ MICHAEL CULHANE
                                           -----------------------------------
                                           Name: Michael Culhane
                                           Title: Vice President

                                       18

<PAGE>   22


STATE OF MASSACHUSETTS  )
COUNTY OF SUFFOLK       )     ss.:

On the 29th day of June, 2000 before me personally came Douglas L. Jacobs, to me
known, who, being by me duly sworn, did depose and say that he resides in
Providence, Rhode Island; that he is Senior Vice President and Treasurer of
FleetBoston Financial Corporation, one of the corporations described in and
which executed the above instrument; that he knows the corporate seal of said
corporation; that the seal affixed to the said instrument is such corporate
seal; that it was so affixed by authority of the Board of Directors of said
corporation; and that he signed his name thereto by like authority.



/s/ PATRICK D. GANNON
----------------------------------
Notary Public

[seal] Commission expires: October 11, 2002





                                       19