<PAGE>   1
                                                                     EXHIBIT 4.2

                           (FORM OF FACE OF SECURITY)

     [IF THIS SECURITY IS A GLOBAL SECURITY INSERT: THIS GLOBAL SECURITY IS A
GLOBAL SECURITY WITHIN THE MEANING OF THE INDENTURE HEREINAFTER REFERRED TO AND
IS REGISTERED IN THE NAME OF THE DEPOSITORY TRUST COMPANY ("DTC") OR A NOMINEE
OF DTC. THIS SECURITY IS EXCHANGEABLE FOR SECURITIES REGISTERED IN THE NAME OF A
PERSON OTHER THAN DTC OR ITS NOMINEE ONLY IN THE LIMITED CIRCUMSTANCES DESCRIBED
IN THE INDENTURE, AND NO TRANSFER OF THIS SECURITY (OTHER THAN A TRANSFER OF
THIS SECURITY AS A WHOLE BY DTC TO A NOMINEE OF DTC OR BY A NOMINEE OF DTC TO
DTC OR ANOTHER NOMINEE OF DTC) MAY BE REGISTERED EXCEPT IN LIMITED
CIRCUMSTANCES.

     UNLESS THIS SECURITY IS PRESENTED BY AN AUTHORIZED REPRESENTATIVE OF DTC TO
THE ISSUER OR ITS AGENT FOR REGISTRATION OF TRANSFER, EXCHANGE OR PAYMENT, AND
ANY SECURITY ISSUED IS REGISTERED IN THE NAME OF CEDE & CO. OR IN SUCH OTHER
NAME AS REQUESTED BY AN AUTHORIZED REPRESENTATIVE OF DTC (AND ANY PAYMENT HEREON
IS MADE TO CEDE & CO. OR TO SUCH OTHER ENTITY AS IS REQUESTED BY AN AUTHORIZED
REPRESENTATIVE OF DTC), ANY TRANSFER, PLEDGE OR OTHER USE HEREOF FOR VALUE OR
OTHERWISE BY OR TO ANY PERSON IS WRONGFUL IN AS MUCH AS THE REGISTERED OWNER
HEREOF, CEDE & CO., HAS AN INTEREST HEREIN.]

     THE SECURITIES WILL BE ISSUED AND MAY BE TRANSFERRED ONLY IN BLOCKS HAVING
AN AGGREGATE PRINCIPAL AMOUNT OF NOT LESS THAN $100,000. ANY ATTEMPTED TRANSFER
OF SECURITIES IN A BLOCK HAVING AN AGGREGATE PRINCIPAL AMOUNT OF LESS THAN
$100,000 AND MULTIPLES OF $1,000 IN EXCESS THEREOF SHALL BE DEEMED TO BE VOID
AND OF NO LEGAL EFFECT WHATSOEVER. ANY SUCH PURPORTED TRANSFEREE SHALL BE DEEMED
NOT TO BE THE HOLDER OF SUCH SECURITIES FOR ANY PURPOSE, INCLUDING, BUT NOT
LIMITED TO, THE RECEIPT OF PRINCIPAL OF OR INTEREST ON SUCH SECURITIES, AND SUCH
PURPORTED TRANSFEREE SHALL BE DEEMED TO HAVE NO INTEREST WHATSOEVER IN SUCH
SECURITIES.

     THE HOLDER OF THIS SECURITY BY ITS ACCEPTANCE HEREOF ALSO AGREES,
REPRESENTS AND WARRANTS THAT IT IS NOT AN EMPLOYEE BENEFIT, INDIVIDUAL
RETIREMENT ACCOUNT OR OTHER PLAN OR ARRANGEMENT SUBJECT TO TITLE I OF THE
EMPLOYEE RETIREMENT INCOME SECURITY ACT OF 1974, AS AMENDED ("ERISA"), OR
SECTION 4975 OF THE INTERNAL REVENUE CODE OF 1986, AS AMENDED (THE "CODE") (EACH
A "PLAN"), OR AN ENTITY WHOSE UNDERLYING ASSETS INCLUDE "PLAN ASSETS" BY REASON
OF ANY PLAN'S INVESTMENT IN THE ENTITY, AND NO PERSON INVESTING "PLAN ASSETS" OF
ANY PLAN MAY ACQUIRE OR HOLD THE SECURITIES OR ANY INTEREST THEREIN, UNLESS SUCH
PURCHASER OR HOLDER IS ELIGIBLE FOR THE


<PAGE>   2


EXEMPTIVE RELIEF AVAILABLE UNDER U.S. DEPARTMENT OF LABOR PROHIBITED TRANSACTION
CLASS EXEMPTION 96-23, 95-60, 91-38, 90-1 OR 84-14 OR ANOTHER APPLICABLE
EXEMPTION OR ITS PURCHASE AND HOLDING OF THIS SECURITY IS NOT PROHIBITED BY
SECTION 406 OF ERISA OR SECTION 4975 OF THE CODE WITH RESPECT TO SUCH PURCHASE
OR HOLDING. ANY PURCHASER OR HOLDER OF THE SECURITIES OR ANY INTEREST THEREIN
WILL BE DEEMED TO HAVE REPRESENTED BY ITS PURCHASE AND HOLDING THEREOF THAT
EITHER (i) IT IS NOT AN EMPLOYEE BENEFIT PLAN WITHIN THE MEANING OF SECTION 3(3)
OF ERISA, OR A PLAN TO WHICH SECTION 4975 OF THE CODE IS APPLICABLE, A TRUSTEE
OR OTHER PERSON ACTING ON BEHALF OF AN EMPLOYEE BENEFIT PLAN OR PLAN, OR ANY
OTHER PERSON OR ENTITY USING THE ASSETS OF ANY EMPLOYEE BENEFIT PLAN OR PLAN TO
FINANCE SUCH PURCHASE, OR (ii) SUCH PURCHASE WILL NOT RESULT IN A PROHIBITED
TRANSACTION UNDER SECTION 406 OF ERISA OR SECTION 4975 OF THE CODE FOR WHICH
THERE IS NO APPLICABLE STATUTORY OR ADMINISTRATIVE EXEMPTION.


<PAGE>   3


                       SUBORDINATED DEBENTURE CERTIFICATE
                             BOSTONFED BANCORP, INC.

CUSIP No.: _____________
      $22,681,000

      10.875% JUNIOR SUBORDINATED DEFERRABLE INTEREST DEBENTURE

                          DUE October 1, 2030 Series B

     BostonFed Bancorp, Inc., a Delaware corporation (the "Corporation," which
term includes any successor Person under the Indenture hereinafter referred to),
for value received, hereby promises to pay to BFD Preferred Capital Trust II or
registered assigns, the principal sum of $22,681,000 (Twenty-Two Million, Six
Hundred Eighty One Thousand Dollars) on October 1, 2030 (the "Maturity Date"),
unless previously prepaid, and to pay interest on the outstanding principal
amount hereof from September 22, 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, semi-annually (subject to deferral as set forth
herein) in arrears on April 1 and October 1 of each year, commencing April 1,
2001 at the rate of 10.875% per annum, until the principal hereof shall have
become due and payable, and on any overdue principal 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 semi-annually ("Compounded Interest"). The amount of interest payable
hereon shall be computed on the basis of a 360-day year of twelve 30-day months.
In the event that any date on which the principal of or interest on this
Security is payable is not a Business Day (as defined in the Indenture), then
the payment payable on such date will be made on the next succeeding day that is
a Business Day, except that if such next succeeding Business Day falls in the
next succeeding calendar year such payment shall be made on the immediately
preceding Business Day (and without any interest or other payment in respect of
any such delay), with the same force and effect as if made on such date.
Pursuant to the Indenture, in certain circumstances the Corporation will be
required to pay Additional Sums (as defined in the Indenture) with respect to
this Security. Pursuant to the Registration Rights Agreement and the Liquidated
Damages Agreement, in certain limited circumstances the Corporation will be
required to pay Liquidated Damages (as defined in the Registration Rights
Agreement and the Liquidated Damages Agreement) with respect to this Security.

     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 Security (or one or more Predecessor Securities,
as defined in said Indenture) is registered at the close of business on the
regular record date for such interest installment, which shall be at the close
of business on the 15th day of the month immediately preceding the month in
which the relevant Interest Payment Date falls. Any such interest installment
not punctually paid or duly provided for shall forthwith cease to be payable to
the holders on such regular record date and may be paid to the Person in whose
name this Security (or one or more Predecessor Securities) is registered at the
close of business on a special record date to be fixed by the Debenture Trustee
for the payment of such defaulted interest, notice whereof shall be given to the
holders of Securities not less than 10 days


<PAGE>   4



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 Securities 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 interest (including Compounded Interest and Additional
Sums, if any) and Liquidated Damages, if any, on this Security shall be payable
at the office or agency of the Debenture 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 Corporation by: (i) check
mailed to the holder at such address as shall appear in the Security Register;
or (ii) transfer to an account maintained by the Person entitled thereto,
provided that proper written transfer instructions have been received by the
relevant record date. Notwithstanding the foregoing, so long as the holder of
this Security is the Property Trustee of BFD Preferred Capital Trust II, the
payment of the principal of and interest (including Compounded Interest and
Additional Sums, if any) and Liquidated Damages, if any, on this Security will
be made at such place and to such account as may be designated by such Property
Trustee.

     The indebtedness evidenced by this Security 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 this Security is issued subject to the
provisions of the Indenture with respect thereto. Each holder of this Security,
by accepting the same, (a) agrees to and shall be bound by such provisions, (b)
authorizes and directs the Debenture 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 Debenture 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, whether now outstanding or hereafter incurred, and waives
reliance by each such holder upon said provisions.

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

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

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     IN WITNESS WHEREOF, the Corporation has caused this instrument to be duly
executed and sealed this _______ day of ______________, 2001.


                                    BOSTONFED BANCORP, INC.



                                    By:
                                       ---------------------------------------
                                       John A. Simas
                                       Executive Vice President and
                                        Corporate Secretary


Attest:

By:
     -------------------------------
       Name:
       Title:





                          CERTIFICATE OF AUTHENTICATION

     This is one of the 10.875% Junior Subordinated Deferrable Interest
Debentures, Series B, of BostonFed Bancorp, Inc. referred to in the
within-mentioned Indenture.


                                    WILMINGTON TRUST COMPANY,
                                    not in its individual capacity but solely
                                    as Debenture Trustee


Dated:                              By:
         ------------------            --------------------------------------
                                       Authorized Signatory


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                          (FORM OF REVERSE OF SECURITY)
                         10.875% SUBORDINATED DEBENTURE

     This Security is one of the Securities of the Corporation (herein sometimes
referred to as the "Securities"), specified in the Indenture, all issued or to
be issued under and pursuant to an Indenture, dated as of September 22, 2000
(the "Indenture"), duly executed and delivered between the Corporation and
Wilmington Trust Company, as Debenture Trustee (the " Debenture Trustee"), to
which Indenture reference is hereby made for a description of the rights,
limitations of rights, obligations, duties and immunities thereunder of the
Debenture Trustee, the Corporation and the holders of the Securities. The
Securities are limited in aggregate principal amount as specified in the
Indenture.

     Upon the occurrence and continuation of a Special Event (as defined in the
Indenture) prior to October 1, 2010 (the "Initial Optional Redemption Date"),
the Corporation shall have the right, at any time within 90 days following the
occurrence of such Special Event, to prepay this Security in whole (but not in
part) at the Special Event Prepayment Price. "Special Event Prepayment Price"
shall mean, with respect to any prepayment of the Securities following a Special
Event, an amount in cash equal to 100% of the principal amount of the Securities
to be prepaid plus any accrued and unpaid interest (including Compounded
Interest and Additional Sums, if any) and Liquidated Damages, if any, thereon to
the date of such prepayment.

     In addition, the Corporation shall have the right to prepay this Security,
in whole or in part, at any time on or after the Initial Optional Redemption
Date (an "Optional Prepayment"), at a price equal to 100% of the principal
amount of this Security plus, in each case, accrued and unpaid interest thereon
(including Compounded Interest and Additional Sums, if any] and Liquidated
Damages, if any, thereon to the applicable date of prepayment (the "Optional
Prepayment Price").

     The Prepayment Price shall be paid prior to 12:00 noon, New York City time,
on the date of such prepayment or at such earlier time as the Corporation
determines, PROVIDED that the Corporation shall deposit with the Debenture
Trustee an amount sufficient to pay the Prepayment Price by 10:00 a.m., New York
City time, on the date such Prepayment Price is to be paid. Any prepayment
pursuant to this paragraph will be made upon not less than 30 days nor more than
60 days' prior written notice.

     If the Securities are only partially prepaid by the Corporation pursuant to
an Optional Prepayment, the particular Securities to be prepaid shall be
selected on a pro rata basis from the outstanding Securities not previously
called for prepayment; PROVIDED, HOWEVER, that with respect to Security holders
that would be required to hold Securities with an aggregate principal amount of
less than $100,000 but more than an aggregate principal amount of zero as a
result of such pro rata prepayment, the Corporation shall prepay Securities of
each such Security holder so that after such prepayment such Security holder
shall hold Securities either with an aggregate principal amount of at least
$100,000 or such Security holder no longer holds any Securities and shall use
such method (including, without limitation, by lot) as the Corporation shall
deem fair and appropriate; PROVIDED, FURTHER, that any such proration may be
made on the basis of the aggregate principal amount of Securities held by each
Security holder thereof and may be made by making such adjustments as the
Corporation deems fair and appropriate in order that only Securities in
denominations of $1,000 or integral multiples thereof shall be prepaid. In the
event of prepayment of this Security in part only, a new Security or Securities
for the portion hereof that has not been prepaid will be issued in the

<PAGE>   7


name of the holder hereof upon the cancellation hereof.

     Notwithstanding the foregoing, any prepayment of Securities by the
Corporation shall be subject to the receipt of any and all required regulatory
approvals.

     In case an Event of Default (as defined in the Indenture) shall have
occurred and be continuing, the principal of all of the Securities 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 Corporation and the
Debenture Trustee, with the consent of the holders of a majority in aggregate
principal amount of the Securities 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 modifying in any manner the rights of the holders of the
Securities; PROVIDED, HOWEVER, that no such supplemental indenture shall,
without the consent of each holder of Securities then outstanding and affected
thereby: (i) change the Maturity Date of any Security, or reduce the rate or
extend the time of payment of interest thereon (subject to Article XVI of the
Indenture), or reduce the principal amount thereof, or change any of the
prepayment provisions or make the principal thereof or any interest thereon
payable in any coin or currency other than U.S. dollars, or impair or affect the
right of any holder of Securities to institute suit for payment thereof; or (ii)
reduce the aforesaid percentage of Securities the holders of which are required
to consent to any such supplemental indenture. The Indenture also contains
provisions permitting the holders of a majority in aggregate principal amount of
the Securities at the time outstanding affected thereby, on behalf of all of the
holders of the Securities, to waive any past default in the performance of any
of the covenants contained in the Indenture, or established pursuant to the
Indenture, and its consequences, except a default in the payment of the
principal of or interest on any of the Securities or a default in respect of any
covenant or provision under which the Indenture cannot be modified or amended
without the consent of each holder of Securities then outstanding. Any such
consent or waiver by the holder of this Security (unless revoked as provided in
the Indenture) shall be conclusive and binding upon such holder and upon all
future holders and owners of this Security and of any Security issued in
exchange herefor or in 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 Security.

     No reference herein to the Indenture and no provision of this Security or
of the Indenture shall alter or impair the obligation of the Corporation, which
is absolute and unconditional, to pay the principal of and interest (including
Compounded Interest and Additional Sums, if any) and Liquidated Damages, if any,
on this Security at the time and place and at the rate and in the money herein
prescribed.

     So long as no Event of Default shall have occurred and be continuing, the
Corporation shall have the right, at any time and from time to time during the
term of the Securities, to defer payments of interest by extending the interest
payment period (an "Extended Interest Payment Period") of such Securities for a
period not: (i) exceeding 10 consecutive semi-annual periods, including the
first such semi-annual period during such extension period; (ii) extending
beyond the Maturity Date of the Securities; or (iii) ending on a date other than
an Interest Payment Date, at the end of which period the Corporation shall pay
all interest then accrued and unpaid (together with interest thereon


<PAGE>   8


at the rate specified for the Securities to the extent that payment of such
interest is enforceable under applicable law). Before the termination of any
such Extended Interest Payment Period, the Corporation may further defer
payments of interest by further extending such Extended Interest Payment Period,
PROVIDED that such Extended Interest Payment Period, together with all such
previous and further extensions within such Extended Interest Payment Period,
(i) shall not exceed 10 consecutive semi-annual periods including the first
semi-annual period during such Extended Interest Payment Period, (ii) shall not
end on any date other than an Interest Payment Date, and (iii) shall not extend
beyond the Maturity Date of the Securities. Upon the termination of any such
Extended Interest Payment Period and the payment of all accrued and unpaid
interest and any additional amounts then due, the Corporation 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 Corporation may prepay at any time all or any
portion of the interest accrued during an Extended Interest Payment Period.

     The Corporation has agreed that it will not (i) declare or pay any
dividends or distributions on, or redeem, purchase, acquire, or make a
liquidation payment with respect to, any of the Corporation's capital stock,
(ii) make any payment of principal of or interest on or repay, repurchase or
redeem any debt securities of the Corporation (including Other Debentures) that
rank pari passu with or junior in right of payment to the Securities or (iii)
make any guarantee payments with respect to any guarantee (other than the
Capital Securities Guarantee) by the Corporation of the debt securities of any
Subsidiary of the Corporation (including Other Guarantees) if such guarantee
ranks pari passu with or junior in right of payment to the Securities (other
than (a) dividends or distributions in shares of, or options, warrants or rights
to subscribe for or purchase shares of, Common Stock, (b) any declaration of a
dividend in connection with the implementation of a stockholders' rights plan,
or the issuance of stock under any such plan in the future, or the redemption or
repurchase of any such rights pursuant thereto, (c) payments under the Capital
Securities Guarantee, as defined in the Indenture, (d) as a result of a
reclassification of the Corporation's capital stock or the exchange or
conversion of one class or series of the Corporation's capital stock for another
class or series of the Corporation's capital stock, (e) the purchase of
fractional interests in shares of the Corporation's capital stock pursuant to
the conversion or exchange provisions of such capital stock or the security
being converted or exchanged and (f) purchases of Common Stock related to the
issuance of Common Stock or rights under any of the Corporation's benefit or
compensation plans for its directors, officers or employees or any of the
Corporation's dividend reinvestment plans), if at such time (1) there shall have
occurred any event of which the Corporation has actual knowledge that (a) is a
Default or an Event of Default and (b) in respect of which the Corporation shall
not have taken reasonable steps to cure, (2) such Securities are held by the
Property Trustee of BFD Preferred Capital Trust II and the Corporation shall be
in default with respect to its payment of any obligations under the Capital
Securities Guarantee or (3) the Corporation shall have given notice of its
election to exercise its right to commence an Extended Interest Payment Period
and shall not have rescinded such notice, and such Extended Interest Payment
Period or any extension thereof shall have commenced and be continuing.

     Subject to (i) the Corporation having received any required regulatory
approvals and (ii) the Administrative Trustees of BFD Preferred Capital Trust II
having received an opinion of counsel to the effect that such distribution will
not cause the holders of Capital Securities to recognize gain or loss for
federal income tax purposes, the Corporation will have the right at any time to
liquidate the Trust and, after satisfaction of liabilities of creditors of the
Trust as required by applicable law, to cause the Securities to be distributed
to the holders of the Trust Securities in liquidation of the Trust.

<PAGE>   9

     The Securities are issuable only in registered form without coupons in
minimum denominations of $100,000 and multiples of $1,000 in excess thereof. As
provided in the Indenture and subject to the transfer restrictions limitations
as may be contained herein and therein from time to time, this Security is
transferable by the holder hereof on the Security Register of the Corporation,
upon surrender of this Security for registration of transfer at the office or
agency of the Corporation in Wilmington, Delaware accompanied by a written
instrument or instruments of transfer in form satisfactory to the Corporation or
the Debenture Trustee duly executed by the holder hereof or his or her attorney
duly authorized in writing, and thereupon one or more new Securities of
authorized denominations and for the same aggregate principal amount will be
issued to the designated transferee or transferees. No service charge will be
made for any such registration of transfer, but the Corporation 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 Security, the
Corporation, the Debenture Trustee, any authenticating agent, any paying agent,
any transfer agent and the security registrar may deem and treat the holder
hereof as the absolute owner hereof (whether or not this Security shall be
overdue and notwithstanding any notice of ownership or writing hereon made by
anyone other than the security registrar for the Securities) for the purpose of
receiving payment of or on account of the principal hereof and (subject to the
Indenture) interest due hereon and for all other purposes, and neither the
Corporation nor the Debenture Trustee nor any authenticating agent nor any
paying agent nor any transfer agent nor 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 interest
(including Compounded Interest and Additional Sums, if any) or Liquidated
Damages, if any, on this Security, or for any claim based hereon, or otherwise
in respect hereof, or based on or in respect of the Indenture, against any
incorporator, stockholder, employee, officer or director, past, present or
future, as such, of the Corporation or of any predecessor or successor Person,
whether by virtue of any constitution, statute or rule of law, or by the
enforcement of any assessment or 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.

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

     THE INDENTURE AND THE SECURITIES SHALL BE GOVERNED BY AND CONSTRUED IN
ACCORDANCE WITH THE LAWS OF THE STATE OF NEW YORK WITHOUT REGARD TO CONFLICT OF
LAW PRINCIPLES THEREOF.