<PAGE>

                                                                  EXHIBIT 4 (ii)

                           MERRILL LYNCH & CO., INC.

                                      TO

                THE CHASE MANHATTAN BANK (NATIONAL ASSOCIATION)
                             as Successor Trustee
                                      and
                      CHEMICAL BANK, as Resigning Trustee
                  (each with respect to the series designated
                             "Medium-Term Notes")

                   ________________________________________

                         SIXTH SUPPLEMENTAL INDENTURE

                         Dated as of October 25, 1993

                   ________________________________________

                           Supplemental to Indenture
                           Dated as of April 1, 1983
                                  as Amended
<PAGE>

SIXTH SUPPLEMENTAL INDENTURE, dated as of October 25, 1993, to the Indenture (as
defined below), by and among MERRILL LYNCH & CO., INC., a corporation duly
organized and existing under the laws of the State of Delaware (the "Company"),
having its principal office at the World Financial Center, New York, New York
10080, THE CHASE MANHATTAN BANK (NATIONAL ASSOCIATION), a national banking
association duly organized and existing under the laws of the United States of
America (the "Successor Trustee"), having its Corporate Trust Office at 4 Chase
MetroTech Center, Brooklyn, New York 11245, and CHEMICAL BANK, a corporation
duly organized and existing under the laws of the State of New York and
successor by merger to MANUFACTURERS HANOVER TRUST COMPANY (the "Resigning
Trustee"), having its Corporate Trust Office at 450 West 33rd Street, New York,
New York 10001.

                            RECITALS OF THE COMPANY

     The Company has heretofore executed and delivered its Indenture, dated as
of April 1, 1983 (as amended to the date hereof, the "Indenture") to the
Resigning Trustee to provide for the issuance from time to time of its unsecured
and unsubordinated debentures, notes or other evidences of senior indebtedness,
including a series of Securities designated the Company's Medium-Term Notes (the
"Medium-Term Notes"). The term "Indenture" shall mean such Indenture as amended
by the Trust Indenture Reform Act of 1990.

     Section 610(b) of the Indenture provides that the Trustee may resign at any
time with respect to the Securities of one or more series, and that should such
resignation occur, the Company, by a Board Resolution, pursuant to Section
610(e), shall authorize the appointment of a successor Trustee with respect to
the Securities of that or those series.

     Section 610(a) of the Indenture provides that no resignation of the
retiring Trustee shall become effective until the acceptance of appointment by a
successor Trustee.

     Sections 611(b) and 901(5) of the Indenture provide that the Company, the
retiring Trustee, and each successor Trustee with respect to the Securities of
such one or more series, when authorized by a Board Resolution, shall execute
and deliver an indenture supplemental to the Indenture, without the consent of
any Holders, to evidence and provide for the acceptance of appointment hereunder
by a successor Trustee with respect to the securities of one or more series.

     The Resigning Trustee has notified the Company of its intention to resign
as Trustee with respect to the series of Securities designated Medium-Term Notes
issued under the Indenture, and the Successor Trustee has indicated its
willingness to accept such appointment as successor Trustee with respect to such
series.

     The Company deems it advisable to supplement the indenture to provide for
such resignation and the successorship of the Successor Trustee with respect to
the series of Securities designated Medium-Term Notes.
<PAGE>

     The Company has duly authorized the execution and delivery of this Sixth
Supplemental Indenture, and all actions necessary to make this Sixth
Supplemental Indenture a valid agreement of the Company, in accordance with its
terms, have been done.

     NOW, THEREFORE, the Company, the Successor Trustee and the Resigning
Trustee, for and in consideration of the premises and of other good and valuable
consideration, the receipt and sufficiency of which are hereby acknowledged,
hereby covenant and agree, for the equal and proportionate benefit of all
Holders, as follows:

                                   ARTICLE I
                                   ---------

                             THE RESIGNING TRUSTEE
                             ---------------------

     1.   Pursuant to Section 610(b) of the Indenture, the Resigning Trustee
hereby notifies the Company that the Resigning Trustee is hereby resigning as
Trustee under the Indenture with respect to the series of Securities designated
Medium-Term Notes.

     2.   The Resigning Trustee hereby assigns, transfers, delivers and confirms
to the Successor Trustee (i) all the rights, title and interest of the Resigning
Trustee in and to the trust under the Indenture with respect to the series of
Securities designated Medium-Term Notes, (ii) all the rights, powers, trusts and
duties of the Trustee under the Indenture with respect to the series of
Securities designated Medium-Term Notes, and (iii) all property and money, if
any, held by the Resigning Trustee under the Indenture with respect to the
series of Securities designated Medium-Term Notes. The Resigning Trustee shall
execute and deliver such further instruments and shall do such other things as
the Successor Trustee may reasonably require so as to more fully and certainly
vest and confirm in the Successor Trustee, all the rights, powers, trusts and
duties hereby assigned, transferred, delivered and confirmed to the Successor
Trustee.

     3.   The Resigning Trustee hereby acknowledges and confirms that all of the
rights, powers, trusts and duties of the Resigning Trustee with respect to all
series of Securities issued under the Indenture other than those designated
Medium-Term Notes shall continue and remain vested in the Resigning Trustee.

     SECTION 104. The Resigning Trustee hereby resigns as Paying Agent with
respect to the series of Securities designated Medium-Term Notes, as Registrar
with respect to the series of Securities designated Medium-Term Notes, and as
the office or agency maintained by the Company pursuant to Section 1002 of the
Indenture with respect to the series of Securities designated Medium-Term Notes.

                                  ARTICLE II
                                  ----------

                                  THE COMPANY
                                  -----------

     1.   The Company hereby accepts the resignation of the Resigning Trustee as
Trustee under the Indenture with respect to the series of Securities designated
Medium-Term Notes.

                                       3
<PAGE>

     2.   The Secretary or Assistant Secretary of the Company who is attesting
to the execution of this Agreement by the Company hereby certifies that Exhibit
A annexed hereto is a copy of the Board Resolutions which were duly adopted by
the Board of Directors of the Company, which resolutions are in full force and
effect on the date hereof, and which authorized certain officers of the Company
to (a) appoint the Successor Trustee as Trustee under the Indenture with respect
to the series of Securities designated Medium-Term Notes, and (b) execute and
deliver such agreements and other instruments as may be necessary or desirable
to effectuate the succession of the Successor Trustee as Trustee under the
Indenture with respect to the series of Securities designated Medium-Term Notes.

     3.   The Company hereby appoints the Successor Trustee as Trustee under the
Indenture with respect to the series of Securities designated Medium-Term Notes
to succeed to, and hereby confirms to, the Successor Trustee, all the rights,
powers, trusts and duties of the Resigning Trustee under the Indenture with
respect to the series of Securities designated Medium-Term Notes with like
effect as if originally named as Trustee in the Indenture with respect to the
Securities of such series. The Company shall execute and deliver such further
instruments and such other things as the Successor Trustee may reasonably
require so as to more fully and certainly vest and confirm in the Successor
Trustee, all the rights, powers, trusts and duties hereby assigned, transferred,
delivered and confirmed to the Successor Trustee.

     4.   Promptly after the effectiveness of this Sixth Supplemental Indenture,
the Company shall cause a notice, substantially in the form of Exhibit B annexed
hereto, to be sent to each Holder of the Medium-Term Notes in accordance with
the provisions of Section 610(f) of the Indenture.

     5.   The Company represents and warrants to the Resigning Trustee and to
the Successor Trustee that, to the best of its knowledge, no event has occurred
and is continuing which is, or after notice or lapse of time or both would
become, an Event of Default under Section 501 of the Indenture.

     6.   The Company hereby acknowledges and confirms that all of the rights,
powers, trusts and duties of the Resigning Trustee with respect to all series of
Securities issued under the Indenture other than those designated Medium-Term
Notes shall continue and remain vested in the Resigning Trustee.

     7.   The Company hereby appoints the Successor Trustee as Paying Agent with
respect to the series of Securities designated Medium-Term Notes, as Registrar
with respect to the series of Securities designated Medium-Term Notes, and as
the Company's office and agency maintained pursuant to Section 1002 of the
Indenture with respect to the series of Securities designated Medium-Term Notes.

                                       4
<PAGE>

                                  ARTICLE III
                                  -----------

                             THE SUCCESSOR TRUSTEE
                             ---------------------

     1.   The Successor Trustee hereby represents and warrants to the Resigning
Trustee and to the Company that the Successor Trustee is not disqualified under
the provisions of Section 608 and is eligible under the provisions of Section
609 of the Indenture to act as Trustee with respect to the series of Securities
designated Medium-Term Notes issued under the Indenture.

     2.   The Successor Trustee hereby accepts its appointment as successor
Trustee under the Indenture with respect to the series of Securities designated
Medium-Term Notes issued under the Indenture and accepts, and shall hereby be
vested with, all the rights, powers, trusts and duties of the Resigning Trustee
as Trustee with respect to the series of Securities designated Medium-Term
Notes, upon the terms and conditions set forth therein, with like effect as if
originally named as Trustee with respect to the Securities of such series under
the Indenture.

     3.   The Successor Trustee hereby acknowledges and confirms that all of the
rights, powers, trusts and duties of the Resigning Trustee with respect to all
series of Securities issued under the Indenture other than those designated
Medium-Term Notes shall continue and remain vested in the Resigning Trustee.

     4.   The Successor Trustee hereby accepts its appointment as Paying Agent
with respect to the series of Securities designated Medium-Term Notes, as
Registrar with respect to the series of Securities designated Medium-Term Notes,
and as the Company's office and agency maintained pursuant to Section 1002 of
the Indenture with respect to the series of Securities designated Medium-Term
Notes.

                                  ARTICLE IV
                                  ----------

                           MISCELLANEOUS PROVISIONS
                           ------------------------

     1.   For all purposes of this Sixth Supplemental Indenture, except as
otherwise stated herein, terms used in capitalized form in this Sixth
Supplemental Indenture and defined in the Indenture have the meanings specified
in the Indenture.

     2.   All of the provisions of the Indenture with respect to the rights,
powers, trusts and duties of each of the Successor Trustee and the Resigning
Trustee, including the provisions of Section 611(b), shall be applicable to each
as fully and with like effect as if set forth herein, except as has been amended
or altered by this Sixth Supplemental Indenture herein.

     3.   This Sixth Supplemental Indenture shall be governed by and construed
in accordance with the laws of the State of New York applicable to agreements
made and to be performed in said State.

                                       5
<PAGE>

     4.   This Sixth Supplemental Indenture may be executed in any number of
counterparts, each of which so executed shall be deemed to be an original, but
all such counterparts shall together constitute but one and the same instrument.

     5.   This Sixth Supplemental Indenture shall become effective as of the
opening of business on the 25th of October, 1993, upon the execution and
delivery hereof by each of the parties hereto.

     6.   The Recitals contained herein shall be taken as the statements of the
Company, and the Resigning Trustee assumes no responsibility for their
correctness. The Resigning Trustee makes no representations as to the validity
or sufficiency of this Sixth Supplemental Indenture.

     7.   Notwithstanding the resignation of the Resigning Trustee effected
hereby, the Company shall remain obligated under Section 607 of the Indenture to
compensate, reimburse and indemnify the Resigning Trustee in connection with its
trusteeship with respect to the series of Securities designated Medium-Term
Notes.

     8.   The Company, the Resigning Trustee and the Successor Trustee hereby
acknowledge receipt of an executed and acknowledged counterpart of this
Agreement and the effectiveness thereof.

     IN WITNESS WHEREOF, the parties hereto have caused this Sixth Supplemental
Indenture to be duly executed, and their respective corporate seals to be
hereunto affixed and attested, all as of the day and year first above written.

                                        MERRILL LYNCH & CO., INC.

[SEAL)

                                        By: _______________________________
                                            Name:
                                            Title:

Attest


____________________________

                                       6
<PAGE>

                                        THE CHASE MANHATTAN BANK
                                        (NATIONAL ASSOCIATION)

(SEAL)
                                                  /s/ James Heaney
                                        By: _______________________________
                                            Name: James Heaney
                                            Title: Vice President

Attest


____________________________



                                        CHEMICAL BANK

(SEAL)
                                                  /s/ F.J. Grippo
                                        By: _______________________________
                                            Name: F.J. Grippo
                                            Title: Vice President

Attest


____________________________

                                       7