<PAGE>

                                                                    EXHIBIT 10.4

                                                            Final Conformed Copy

--------------------------------------------------------------------------------

                               GUARANTEE AGREEMENT

                                     between

                          MERCANTILE BANK CORPORATION,
                                  As Guarantor,

                                       and

                            WILMINGTON TRUST COMPANY,
                              As Guarantee Trustee

                         Dated as of September 16, 2004

                         MERCANTILE BANK CAPITAL TRUST I

--------------------------------------------------------------------------------

<PAGE>

                                TABLE OF CONTENTS

<TABLE>
<S>                                                                            <C>
ARTICLE I     INTERPRETATION AND DEFINITIONS.................................   2
SECTION 1.1   Interpretation.................................................   2
SECTION 1.2   Definitions....................................................   2

ARTICLE II    REPORTS........................................................   6
SECTION 2.1   List of Holders................................................   6
SECTION 2.2   Periodic Reports to the Guarantee Trustee......................   6
SECTION 2.3   Event of Default; Waiver.......................................   6
SECTION 2.4   Event of Default; Notice.......................................   7

ARTICLE III   POWERS, DUTIES AND RIGHTS OF THE GUARANTEE TRUSTEE.............   7
SECTION 3.1   Powers and Duties of the Guarantee Trustee.....................   7
SECTION 3.2   Certain Rights of the Guarantee Trustee........................   8
SECTION 3.3   Compensation...................................................  10
SECTION 3.4   Indemnity......................................................  10
SECTION 3.5   Securities.....................................................  11

ARTICLE IV    GUARANTEE TRUSTEE..............................................  11
SECTION 4.1   Guarantee Trustee; Eligibility.................................  11
SECTION 4.2   Appointment, Removal and Resignation of the Guarantee Trustee..  12

ARTICLE V     GUARANTEE......................................................  12
SECTION 5.1   Guarantee......................................................  12
SECTION 5.2   Waiver of Notice and Demand....................................  13
SECTION 5.3   Obligations Not Affected.......................................  13
SECTION 5.4   Rights of Holders..............................................  14
SECTION 5.5   Guarantee of Payment...........................................  14
SECTION 5.6   Subrogation....................................................  14
SECTION 5.7   Independent Obligations........................................  15
SECTION 5.8   Enforcement....................................................  15

ARTICLE VI    COVENANTS AND SUBORDINATION....................................  15
SECTION 6.1   Dividends, Distributions and Payments..........................  15
SECTION 6.2   Subordination..................................................  16
SECTION 6.3   Pari Passu Guarantees..........................................  16

ARTICLE VII   TERMINATION....................................................  16
SECTION 7.1   Termination....................................................  16

ARTICLE VIII  MISCELLANEOUS..................................................  17
SECTION 8.1   Successors and Assigns.........................................  17
SECTION 8.2   Amendments.....................................................  17
SECTION 8.3   Notices........................................................  17
SECTION 8.4   Benefit........................................................  18
</TABLE>

                                        i

<PAGE>

<TABLE>
<S>                                                                            <C>
SECTION 8.5   Governing Law..................................................  18
SECTION 8.6   Submission to Jurisdiction.....................................  18
SECTION 8.7   Counterparts...................................................  19
</TABLE>

                                       ii

<PAGE>

      GUARANTEE AGREEMENT, dated as of September 16, 2004, executed and
delivered by MERCANTILE BANK CORPORATION, a Michigan corporation (the
"Guarantor") having its principal office at 5650 Byron Center Avenue SW,
Wyoming, Michigan 49509, and WILMINGTON TRUST Company, a Delaware banking
corporation, as trustee (in such capacity, the "Guarantee Trustee"), for the
benefit of the Holders (as defined herein) from time to time of the Preferred
Securities (as defined herein) of Mercantile Bank Capital Trust I, a Delaware
statutory trust (the "Issuer").

                              W I T N E S S E T H :

      WHEREAS, pursuant to an Amended and Restated Trust Agreement, dated as of
the date hereof (the "Trust Agreement"), among the Guarantor, as Depositor, the
Property Trustee, the Delaware Trustee and the Administrative Trustees named
therein and the holders from time to time of the Preferred Securities (as
hereinafter defined), the Issuer is issuing $32,000,000 aggregate Liquidation
Amount (as defined in the Trust Agreement) of its Series A and Series B Floating
Rate Preferred Securities (Liquidation Amount $1,000 per preferred security)
(the "Preferred Securities") representing preferred undivided beneficial
interests in the assets of the Issuer and having the terms set forth in the
Trust Agreement;

      WHEREAS, the Preferred Securities will be issued by the Issuer and the
proceeds thereof, together with the proceeds from the issuance of the Issuer's
Common Securities (as defined below), will be used to purchase the Notes (as
defined in the Trust Agreement) of the Guarantor; and

      WHEREAS, as incentive for the Holders to purchase Preferred Securities the
Guarantor desires irrevocably and unconditionally to agree, to the extent set
forth herein, to pay to the Holders of the Preferred Securities the Guarantee
Payments (as defined herein) and to make certain other payments on the terms and
conditions set forth herein.

      NOW, THEREFORE, in consideration of the purchase by each Holder of
Preferred Securities, which purchase the Guarantor hereby agrees shall benefit
the Guarantor, the Guarantor executes and delivers this Guarantee Agreement to
provide as follows for the benefit of the Holders from time to time of the
Preferred Securities:

                                       1
<PAGE>

                                    ARTICLE I

                         INTERPRETATION AND DEFINITIONS

      SECTION 1.1. Interpretation.

      In this Guarantee Agreement, unless the context otherwise requires:

            (a)   capitalized terms used in this Guarantee Agreement but not
      defined in the preamble hereto have the respective meanings assigned to
      them in Section 1.2;

            (b)   the words "include", "includes" and "including" shall be
      deemed to be followed by the phrase "without limitation";

            (c)   all references to "the Guarantee Agreement" or "this Guarantee
      Agreement" are to this Guarantee Agreement as modified, supplemented or
      amended from time to time;

            (d)   all references in this Guarantee Agreement to Articles and
      Sections are to Articles and Sections of this Guarantee Agreement unless
      otherwise specified;

            (e)   the words "hereby", "herein", "hereof" and "hereunder" and
      other words of similar import refer to this Guarantee Agreement as a whole
      and not to any particular Article, Section or other subdivision;

            (f)   a reference to the singular includes the plural and vice
      versa; and

            (g)   the masculine, feminine or neuter genders used herein shall
      include the masculine, feminine and neuter genders.

      SECTION 1.2 Definitions.

      As used in this Guarantee Agreement, the terms set forth below shall,
unless the context otherwise requires, have the following meanings:

            "Affiliate" of any specified Person means any other Person directly
      or indirectly controlling or controlled by or under direct or indirect
      common control with such specified Person; provided, that the Issuer shall
      not be deemed to be an Affiliate of the Guarantor. For the purposes of
      this definition, "control" when used with respect to any specified Person
      means the power to direct the management and policies of such Person,
      directly or indirectly, whether through the ownership of voting
      securities, by contract or otherwise; and the terms "controlling" and
      "controlled" have meanings correlative to the foregoing.

            "Beneficiaries" means any Person to whom the Issuer is or hereafter
      becomes indebted or liable.

                                       2
<PAGE>

            "Board of Directors" means either the board of directors of the
      Guarantor or any duly authorized committee of that board.

            "Common Securities" means the Series A and Series B Floating Rate
      Common Securities representing common undivided beneficial interests in
      the assets of the Issuer.

            "Debt" means with respect to any Person, whether recourse is to all
      or a portion of the assets of such Person, whether currently existing or
      hereafter incurred, and whether or not contingent and without duplication,
      (i) every obligation of such Person for money borrowed; (ii) every
      obligation of such Person evidenced by bonds, debentures, notes or other
      similar instruments, including obligations incurred in connection with the
      acquisition of property, assets or businesses; (iii) every reimbursement
      obligation of such Person with respect to letters of credit, bankers'
      acceptances or similar facilities issued for the account of such Person;
      (iv) every obligation of such Person issued or assumed as the deferred
      purchase price of property or services (but excluding trade accounts
      payable arising in the ordinary course of business); (v) every capital
      lease obligation of such Person; (vi) all indebtedness of such Person,
      whether incurred on or prior to the date of this Guarantee Agreement or
      thereafter incurred, for claims in respect of derivative products,
      including interest rate, foreign exchange rate and commodity forward
      contracts, options, swaps and similar arrangements; (vii) every obligation
      of the type referred to in clauses (i) through (vi) of another Person and
      all dividends of another Person the payment of which, in either case, such
      Person has guaranteed or is responsible or liable for, directly or
      indirectly, as obligor or otherwise; and (viii) any renewals, extensions,
      refundings, amendments or modifications of any obligation of the type
      referred to in clauses (i) through (vii).

            "Event of Default" means a default by the Guarantor on any of its
      payment or other obligations under this Guarantee Agreement; provided,
      that except with respect to a default in payment of any Guarantee
      Payments, the Guarantor shall have received notice of default from the
      Guarantee Trustee and shall not have cured such default within thirty (30)
      days after receipt of such notice.

            "Guarantee Payments" means the following payments or distributions,
      without duplication, with respect to each series of Preferred Securities,
      to the extent not paid or made by or on behalf of the Issuer: (i) any
      accumulated and unpaid Distributions (as defined in the Trust Agreement)
      required to be paid on a given series of Preferred Securities, to the
      extent the Issuer shall have funds on hand available therefor at such
      time, (ii) the Redemption Price (as defined in the Trust Agreement) with
      respect to a given series of Preferred Securities to the extent the Issuer
      shall have funds on hand available therefor at such time, and (iii) upon a
      voluntary or involuntary termination, winding up or liquidation of the
      Issuer, unless Notes are distributed to the Holders, the lesser of (a) the
      aggregate of the Liquidation Amount of $1,000 per Preferred Security plus
      accumulated and

                                       3
<PAGE>

      unpaid Distributions on each series of Preferred Securities to the date of
      payment, to the extent that the Issuer shall have funds available therefor
      at such time and (b) the amount of assets of the Issuer remaining
      available for distribution to Holders in liquidation of the Issuer after
      satisfaction of liabilities to creditors of the Issuer in accordance with
      applicable law (in either case, the "Liquidation Distribution").

            "Guarantee Trustee" means Wilmington Trust Company, until a
      Successor Guarantee Trustee, as defined below, has been appointed and has
      accepted such appointment pursuant to the terms of this Guarantee
      Agreement, and thereafter means each such Successor Guarantee Trustee.

            "Holder" means any holder, as registered on the books and records of
      the Issuer, of any Preferred Securities; provided, that, in determining
      whether the holders of the requisite percentage of Preferred Securities
      have given any request, notice, consent or waiver hereunder, "Holder"
      shall not include the Guarantor, the Guarantee Trustee or any Affiliate of
      the Guarantor or the Guarantee Trustee.

            "Indenture" means the Junior Subordinated Indenture, dated as of the
      date hereof, as supplemented and amended, between the Guarantor and
      Wilmington Trust Company, as trustee.

            "List of Holders" has the meaning specified in Section 2.1.

            "Majority in Liquidation Amount of the Preferred Securities" means a
      vote by the Holder(s) of a given series, voting separately as a class, of
      more than fifty percent (50%) of the aggregate Liquidation Amount of all
      then outstanding Preferred Securities in that series.

            "Obligations" means any costs, expenses or liabilities (but not
      including liabilities related to taxes) of the Issuer, other than
      obligations of the Issuer to pay to holders of a given series of Preferred
      Securities the amounts due such holders pursuant to the terms of such
      Preferred Securities.

            "Officers' Certificate" means, with respect to any Person, a
      certificate signed by the Chief Executive Officer, Chief Financial
      Officer, President or a Vice President of such Person, and by the
      Treasurer, an Assistant Treasurer, the Secretary or an Assistant Secretary
      of such Person, and delivered to the Guarantee Trustee. Any Officers'
      Certificate delivered with respect to compliance with a condition or
      covenant provided for in this Guarantee Agreement (other than the
      certificate provided pursuant to Section 2.4) shall include:

            (a)   a statement that each officer signing the Officers'
      Certificate has read the covenant or condition and the definitions
      relating thereto;

            (b)   a brief statement of the nature and scope of the examination
      or investigation undertaken by each officer in rendering the Officers'
      Certificate;

                                       4
<PAGE>

            (c)   a statement that each officer has made such examination or
      investigation as, in such officer's opinion, is necessary to enable such
      officer to express an informed opinion as to whether or not such covenant
      or condition has been complied with; and

            (d)   a statement as to whether, in the opinion of each officer,
      such condition or covenant has been complied with.

            "Person" means a legal person, including any individual,
      corporation, estate, partnership, joint venture, association, joint stock
      company, limited liability company, trust, unincorporated association,
      government or any agency or political subdivision thereof or any other
      entity of whatever nature.

            "Responsible Officer" means, with respect to the Guarantee Trustee,
      any Senior Vice President, any Vice President, any Assistant Vice
      President, the Secretary, any Assistant Secretary, the Treasurer, any
      Assistant Treasurer, any Financial Services Officer or Assistant Financial
      Services Officer or any other officer of the Corporate Trust Department of
      the Guarantee Trustee and also means, with respect to a particular
      corporate trust matter, any other officer to whom such matter is referred
      because of that officer's knowledge of and familiarity with the particular
      subject.

            "Senior Debt" means the principal of and any premium and interest on
      (including interest accruing on or after the filing of any petition in
      bankruptcy or for reorganization relating to the Guarantor whether or not
      such claim for post-petition interest is allowed in such proceeding) all
      Debt of the Guarantor, whether incurred on or prior to the date of the
      Indenture or thereafter incurred, unless it is provided in the instrument
      creating or evidencing the same or pursuant to which the same is
      outstanding, that such obligations are not superior in right of payment to
      the Preferred Securities; provided, however, that if the Guarantor is
      subject to the regulation and supervision of an "appropriate Federal
      banking agency" within the meaning of 12 U.S.C. 1813(q), the Guarantor
      shall have received the approval of such appropriate Federal banking
      agency prior to issuing any such obligation if not otherwise generally
      approved; provided further, that Senior Debt shall not include any other
      debt securities, and guarantees in respect of such debt securities, issued
      to any trust other than the Issuer (or a trustee of such trust),
      partnership or other entity affiliated with the Guarantor that is a
      financing vehicle of the Guarantor (a "financing entity"), in connection
      with the issuance by such financing entity of equity securities or other
      securities that are treated as equity capital for regulatory capital
      purposes guaranteed by the Guarantor pursuant to an instrument that ranks
      pari passu with or junior in right of payment to this Guarantee Agreement,
      including, without limitation, securities issued by MBWM Capital Trust I.

            "Successor Guarantee Trustee" means a successor Guarantee Trustee
      possessing the qualifications to act as Guarantee Trustee under Section
      4.1.

                                       5
<PAGE>

            "Trust Indenture Act" means the Trust Indenture Act of 1939, as
      amended and as in effect on the date of this Guarantee Agreement.

Capitalized or otherwise defined terms used but not otherwise defined herein
shall have the meanings assigned to such terms in the Trust Agreement as in
effect on the date hereof.

                                   ARTICLE II

                                     REPORTS

      SECTION 2.1 List of Holders.

      The Guarantor shall furnish or cause to be furnished to the Guarantee
Trustee at such times as the Guarantee Trustee may request in writing, within
thirty (30) days after the receipt by the Guarantor of any such request, a list,
in such form as the Guarantee Trustee may reasonably require, of the names and
addresses of the Holders (the "List of Holders") as of a date not more than
fifteen (15) days prior to the time such list is furnished, in each case to the
extent such information is in the possession or control of the Guarantor and is
not identical to a previously supplied list of Holders or has not otherwise been
received by the Guarantee Trustee in its capacity as such. The Guarantee Trustee
may destroy any List of Holders previously given to it on receipt of a new List
of Holders.

      SECTION 2.2 Periodic Reports to the Guarantee Trustee.

      The Guarantor shall deliver to the Guarantee Trustee, within one hundred
and twenty (120) days after the end of each fiscal year of the Guarantor ending
after the date of this Guarantee Agreement, an Officers' Certificate covering
the preceding fiscal year, stating whether or not to the knowledge of the
signers thereof the Guarantor is in default in the performance or observance of
any of the terms or provisions or any of the conditions of this Guarantee
Agreement (without regard to any period of grace or requirement of notice
provided hereunder) and, if the Guarantor shall be in default thereof,
specifying all such defaults and the nature and status thereof of which they
have knowledge.

      SECTION 2.3 Event of Default; Waiver.

      The Holders of a Majority in Liquidation Amount of the Preferred
Securities may, on behalf of the Holders of the applicable series of Preferred
Securities, waive any past Event of Default and its consequences. Upon such
waiver, any such Event of Default with respect to that series shall cease to
exist, and any Event of Default arising therefrom shall be deemed to have been
cured, for every purpose of this Guarantee Agreement, but no such waiver shall
extend to any subsequent or other default or Event of Default or impair any
right consequent therefrom.

                                       6
<PAGE>

      SECTION 2.4 Event of Default; Notice.

            (a)   The Guarantee Trustee shall, within ninety (90) days after the
      occurrence of a default, transmit to the Holders of any affected series of
      Preferred Securities notices of all defaults actually known to the
      Guarantee Trustee, unless such defaults have been cured or waived before
      the giving of such notice, provided, that, except in the case of a default
      in the payment of a Guarantee Payment, the Guarantee Trustee shall be
      protected in withholding such notice if and so long as the Board of
      Directors, the executive committee or a trust committee of directors
      and/or Responsible Officers of the Guarantee Trustee in good faith
      determine that the withholding of such notice is in the interests of the
      Holders. For the purpose of this Section 2.4, the term "default" means any
      event that is, or after notice or lapse of time or both would become, an
      Event of Default.

            (b)   The Guarantee Trustee shall not be deemed to have knowledge of
      any Event of Default unless the Guarantee Trustee shall have received
      written notice, or a Responsible Officer charged with the administration
      of this Guarantee Agreement shall have obtained written notice, of such
      Event of Default from the Guarantor or a Holder.

                                   ARTICLE III

               POWERS, DUTIES AND RIGHTS OF THE GUARANTEE TRUSTEE

            SECTION 3.1 Powers and Duties of the Guarantee Trustee.

            (a)   This Guarantee Agreement shall be held by the Guarantee
      Trustee for the benefit of the Holders, and the Guarantee Trustee shall
      not transfer this Guarantee Agreement to any Person except a Holder
      exercising its rights pursuant to Section 5.4(d) or to a Successor
      Guarantee Trustee upon acceptance by such Successor Guarantee Trustee of
      its appointment to act as Successor Guarantee Trustee. The right, title
      and interest of the Guarantee Trustee shall automatically vest in any
      Successor Guarantee Trustee, upon acceptance by such Successor Guarantee
      Trustee of its appointment hereunder, and such vesting and cessation of
      title shall be effective whether or not conveyancing documents have been
      executed and delivered pursuant to the appointment of such Successor
      Guarantee Trustee.

            (b)   The rights, immunities, duties and responsibilities of the
      Guarantee Trustee shall be as provided by this Guarantee Agreement and
      there shall be no other duties or obligations, express or implied, of the
      Guarantee Trustee. Notwithstanding the foregoing, no provisions of this
      Guarantee Agreement shall require the Guarantee Trustee to expend or risk
      its own funds or otherwise incur any financial liability in the
      performance of any of its duties hereunder, or in the exercise of any of
      its rights or powers, if it shall have reasonable grounds for believing
      that repayment of such funds or adequate indemnity against such risk or
      liability is not reasonably assured to it. Whether or not herein expressly
      so

                                       7
<PAGE>

      provided, every provision of this Guarantee Agreement relating to the
      conduct or affecting the liability of or affording protection to the
      Guarantee Trustee shall be subject to the provisions of this Section 3.1.
      To the extent that, at law or in equity, the Guarantee Trustee has duties
      and liabilities relating to the Guarantor or the Holders, the Guarantee
      Trustee shall not be liable to any Holder for the Guarantee Trustee's good
      faith reliance on the provisions of this Guarantee Agreement. The
      provisions of this Guarantee Agreement, to the extent that they restrict
      the duties and liabilities of the Guarantee Trustee otherwise existing at
      law or in equity, are agreed by the Guarantor and the Holders to replace
      such other duties and liabilities of the Guarantee Trustee.

            (c)   No provision of this Guarantee Agreement shall be construed to
      relieve the Guarantee Trustee from liability for its own negligent action,
      negligent failure to act or own willful misconduct, except that:

                  (i)    the Guarantee Trustee shall not be liable for any error
            of judgment made in good faith by a Responsible Officer of the
            Guarantee Trustee, unless it shall be proved that the Guarantee
            Trustee was negligent in ascertaining the pertinent facts upon which
            such judgment was made; and

                  (ii)   the Guarantee Trustee shall not be liable with respect
            to any action taken or omitted to be taken by it in good faith in
            accordance with the direction of the Holders of not less than a
            Majority in Liquidation Amount of the Preferred Securities relating
            to the time, method and place of conducting any proceeding for any
            remedy available to the Guarantee Trustee, or exercising any trust
            or power conferred upon the Guarantee Trustee under this Guarantee
            Agreement.

      SECTION 3.2 Certain Rights of the Guarantee Trustee.

            (a)   Subject to the provisions of Section 3.1:

                  (i)    the Guarantee Trustee may conclusively rely and shall
            be fully protected in acting or refraining from acting in good faith
            and in accordance with the terms hereof upon any resolution,
            certificate, statement, instrument, opinion, report, notice,
            request, direction, consent, order, bond, debenture, note, other
            evidence of indebtedness or other paper or document reasonably
            believed by it to be genuine and to have been signed, sent or
            presented by the proper party or parties;

                  (ii)   any direction or act of the Guarantor contemplated by
            this Guarantee Agreement shall be sufficiently evidenced by an
            Officers' Certificate unless otherwise prescribed herein;

                  (iii)  the Guarantee Trustee may consult with counsel, and the
            advice of such counsel shall be full and complete authorization and

                                       8
<PAGE>

            protection in respect of any action taken, suffered or omitted to be
            taken by it hereunder in good faith and in reliance thereon and in
            accordance with such advice. Such counsel may be counsel to the
            Guarantee Trustee, the Guarantor or any of its Affiliates and may be
            one of its employees. The Guarantee Trustee shall have the right at
            any time to seek instructions concerning the administration of this
            Guarantee Agreement from any court of competent jurisdiction;

                  (iv)   the Guarantee Trustee shall be under no obligation to
            exercise any of the rights or powers vested in it by this Guarantee
            Agreement at the request or direction of any Holder, unless such
            Holder shall have provided to the Guarantee Trustee reasonable
            security or indemnity against the costs, expenses (including
            reasonable attorneys' fees and expenses) and liabilities that might
            be incurred by it in complying with such request or direction,
            including such reasonable advances as may be requested by the
            Guarantee Trustee; provided, that, nothing contained in this Section
            3.2(a)(iv) shall be taken to relieve the Guarantee Trustee, upon the
            occurrence of an Event of Default, of its obligation to exercise the
            rights and powers vested in it by this Guarantee Agreement;

                  (v)    the Guarantee Trustee shall not be bound to make any
            investigation into the facts or matters stated in any resolution,
            certificate, statement, instrument, opinion, report, notice,
            request, direction, consent, order, bond, debenture, note, other
            evidence of indebtedness or other paper or document, but the
            Guarantee Trustee, in its discretion, may make such further inquiry
            or investigation into such facts or matters as it may see fit, and
            if the Guarantee Trustee shall determine to make such inquiry or
            investigation, it shall be entitled to examine the books, records
            and premises of the Guarantor, personally or by agent or attorney;

                  (vi)   the Guarantee Trustee may execute any of the trusts or
            powers hereunder or perform any duties hereunder either directly or
            by or through its agents, attorneys, custodians or nominees and the
            Guarantee Trustee shall not be responsible for any misconduct or
            negligence on the part of any such agent, attorney, custodian or
            nominee appointed with due care by it hereunder;

                  (vii)  whenever in the administration of this Guarantee
            Agreement the Guarantee Trustee shall deem it desirable to receive
            instructions with respect to enforcing any remedy or right
            hereunder, the Guarantee Trustee (A) may request instructions from
            the Holders of a Majority in Liquidation Amount of the Preferred
            Securities, (B) may refrain from enforcing such remedy or right or
            taking such other action until such instructions are received and
            (C) shall be protected in acting in accordance with such
            instructions;

                                       9
<PAGE>

                  (viii) except as otherwise expressly provided by this
            Guarantee Agreement, the Guarantee Trustee shall not be under any
            obligation to take any action that is discretionary under the
            provisions of this Guarantee Agreement; and

                  (ix)   whenever, in the administration of this Guarantee
            Agreement, the Guarantee Trustee shall deem it desirable that a
            matter be proved or established before taking, suffering or omitting
            to take any action hereunder, the Guarantee Trustee (unless other
            evidence is herein specifically prescribed) may, in the absence of
            bad faith on its part, request and rely upon an Officers'
            Certificate which, upon receipt of such request from the Guarantee
            Trustee, shall be promptly delivered by the Guarantor.

            (b)   No provision of this Guarantee Agreement shall be deemed to
      impose any duty or obligation on the Guarantee Trustee to perform any act
      or acts or exercise any right, power, duty or obligation conferred or
      imposed on it in any jurisdiction in which it shall be illegal, or in
      which the Guarantee Trustee shall be unqualified or incompetent in
      accordance with applicable law, to perform any such act or acts or to
      exercise any such right, power, duty or obligation. No permissive power or
      authority available to the Guarantee Trustee shall be construed to be a
      duty to act in accordance with such power and authority.

      SECTION 3.3 Compensation.

      The Guarantor agrees to pay to the Guarantee Trustee from time to time
reasonable compensation for all services rendered by it hereunder (which
compensation shall not be limited by any provisions of law in regard to the
compensation of a trustee of an express trust) and to reimburse the Guarantee
Trustee upon request for all reasonable expenses, disbursements and advances
(including the reasonable fees and expenses of its attorneys and agents)
incurred or made by the Guarantee Trustee in accordance with any provisions of
this Guarantee Agreement.

      SECTION 3.4 Indemnity.

      The Guarantor agrees to indemnify and hold harmless the Guarantee Trustee
and any of its Affiliates and any of their officers, directors, shareholders,
employees, representatives or agents from and against any loss, damage,
liability, tax (other than income, franchise or other taxes imposed on amounts
paid pursuant to Section 3.3), penalty, expense or other claim of any kind or
nature whatsoever incurred without negligence, bad faith or willful misconduct
on the part of the Guarantee Trustee, any of its Affiliates, or any of their
officers, directors, shareholders, employees, representatives or agents arising
out of or in connection with the acceptance or administration of this Guarantee
Agreement, including the costs and expenses of defending itself against any
claim or liability in connection with the exercise or performance of any of its
powers or duties hereunder. The Guarantee Trustee will not claim or exact any
lien or charge on any Guarantee Payments as a result of any amount due to it
under this Guarantee

                                       10
<PAGE>

Agreement. This indemnity shall survive the termination of this Agreement or the
resignation or removal of the Guarantee Trustee.

      In no event shall the Guarantee Trustee be liable for any indirect,
special, punitive or consequential loss or damage of any kind whatsoever,
including, but not limited to, lost profits, even if the Guarantee Trustee has
been advised of the likelihood of such loss or damage and regardless of the form
of action.

      In no event shall the Guarantee Trustee be liable for any failure or delay
in the performance of its obligations hereunder because of circumstances beyond
its control, including, but not limited to, acts of God, flood, war (declared or
undeclared), terrorism, fire, riot, embargo or government action, including any
laws, ordinances, regulations, governmental action or the like which delay,
restrict or prohibit the providing of the services contemplated by this
Guarantee Agreement.

      SECTION 3.5 Securities.

      The Guarantee Trustee or any other agent of the Guarantee Trustee, in its
individual or any other capacity, may become the owner or pledgee of Common or
Preferred Securities.

                                   ARTICLE IV

                                GUARANTEE TRUSTEE

      SECTION 4.1 Guarantee Trustee; Eligibility.

            (a)   There shall at all times be a Guarantee Trustee which shall:

                  (i)    not be an Affiliate of the Guarantor; and

                  (ii)   be a corporation organized and doing business under the
            laws of the United States or of any State thereof, authorized to
            exercise corporate trust powers, having a combined capital and
            surplus of at least fifty million dollars ($50,000,000), subject to
            supervision or examination by Federal or State authority and having
            an office within the United States. If such corporation publishes
            reports of condition at least annually, pursuant to law or to the
            requirements of such supervising or examining authority, then, for
            the purposes of this Section 4.1, the combined capital and surplus
            of such corporation shall be deemed to be its combined capital and
            surplus as set forth in its most recent report of condition so
            published.

            (b)   If at any time the Guarantee Trustee shall cease to be
      eligible to so act under Section 4.1(a), the Guarantee Trustee shall
      immediately resign in the manner and with the effect set out in Section
      4.2(c).

            (c)   If the Guarantee Trustee has or shall acquire any "conflicting
      interest" within the meaning of Section 310(b) of the Trust Indenture Act,
      the

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      Guarantee Trustee shall either eliminate such interest or resign in the
      manner and with the effect set out in Section 4.2(c).

      SECTION 4.2 Appointment, Removal and Resignation of the Guarantee Trustee.

            (a)   Subject to Section 4.2(b), the Guarantee Trustee may be
      appointed or removed without cause at any time by the Guarantor, except
      during an Event of Default.

            (b)   The Guarantee Trustee shall not be removed until a Successor
      Guarantee Trustee has been appointed and has accepted such appointment by
      written instrument executed by such Successor Guarantee Trustee and
      delivered to the Guarantor.

            (c)   The Guarantee Trustee appointed hereunder shall hold office
      until a Successor Guarantee Trustee shall have been appointed or until its
      removal or resignation. The Guarantee Trustee may resign from office
      (without need for prior or subsequent accounting) by an instrument in
      writing executed by the Guarantee Trustee and delivered to the Guarantor,
      which resignation shall not take effect until a Successor Guarantee
      Trustee has been appointed and has accepted such appointment by instrument
      in writing executed by such Successor Guarantee Trustee and delivered to
      the Guarantor and the resigning Guarantee Trustee.

            (d)   If no Successor Guarantee Trustee shall have been appointed
      and accepted appointment as provided in this Section 4.2 within thirty
      (30) days after delivery to the Guarantor of an instrument of resignation,
      the resigning Guarantee Trustee may petition, at the expense of the
      Guarantor, any court of competent jurisdiction for appointment of a
      Successor Guarantee Trustee. Such court may thereupon, after prescribing
      such notice, if any, as it may deem proper, appoint a Successor Guarantee
      Trustee.

                                    ARTICLE V

                                    GUARANTEE

      SECTION 5.1 Guarantee.

            (a)   The Guarantor irrevocably and unconditionally agrees to pay in
      full to the Holders the Guarantee Payments (without duplication of amounts
      theretofore paid by or on behalf of the Issuer), as and when due,
      regardless of any defense (except for the defense of payment by the
      Issuer), right of set-off or counterclaim which the Issuer may have or
      assert. The Guarantor's obligation to make a Guarantee Payment may be
      satisfied by direct payment of the required amounts by the Guarantor to
      the Holders or by causing the Issuer to pay such amounts to the Holders.
      The Guarantor shall give prompt written notice to the

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      Guarantee Trustee in the event it makes any direct payment to the Holders
      hereunder.

            (b)   The Guarantor hereby also agrees to assume any and all
      Obligations of the Issuer, and, in the event any such Obligation is not so
      assumed, subject to the terms and conditions hereof, the Guarantor hereby
      irrevocably and unconditionally guarantees to each Beneficiary the full
      payment, when and as due, of any and all Obligations to such
      Beneficiaries. This Guarantee is intended to be for the Beneficiaries who
      have received notice hereof.

      SECTION 5.2 Waiver of Notice and Demand.

      The Guarantor hereby waives notice of acceptance of the Guarantee
Agreement and of any liability to which it applies or may apply, presentment,
demand for payment, any right to require a proceeding first against the
Guarantee Trustee, Issuer or any other Person before proceeding against the
Guarantor, protest, notice of nonpayment, notice of dishonor, notice of
redemption and all other notices and demands.

      SECTION 5.3 Obligations Not Affected.

      The obligations, covenants, agreements and duties of the Guarantor under
this Guarantee Agreement shall in no way be affected or impaired by reason of
the happening from time to time of any of the following:

            (a)   the release or waiver, by operation of law or otherwise, of
      the performance or observance by the Issuer of any express or implied
      agreement, covenant, term or condition relating to the Preferred
      Securities to be performed or observed by the Issuer;

            (b)   the extension of time for the payment by the Issuer of all or
      any portion of the Distributions (other than an extension of time for
      payment of Distributions that results from the extension of any interest
      payment period on the Notes as provided in the Indenture), Redemption
      Price, Liquidation Distribution or any other sums payable under the terms
      of a given series of Preferred Securities or the extension of time for the
      performance of any other obligation under, arising out of, or in
      connection with, such Preferred Securities;

            (c)   any failure, omission, delay or lack of diligence on the part
      of the Holders to enforce, assert or exercise any right, privilege, power
      or remedy conferred on the Holders pursuant to the terms of a given series
      of Preferred Securities, or any action on the part of the Issuer granting
      indulgence or extension of any kind;

            (d)   the voluntary or involuntary liquidation, dissolution, sale of
      any collateral, receivership, insolvency, bankruptcy, assignment for the
      benefit of creditors, reorganization, arrangement, composition or
      readjustment of debt of, or other similar proceedings affecting, the
      Issuer or any of the assets of the Issuer;

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<PAGE>

            (e)   any invalidity of, or defect or deficiency in, a given series
      of Preferred Securities;

            (f)   the settlement or compromise of any obligation guaranteed
      hereby or hereby incurred; or

            (g)   any other circumstance whatsoever that might otherwise
      constitute a legal or equitable discharge or defense of a guarantor, it
      being the intent of this Section 5.3 that the obligations of the Guarantor
      hereunder shall be absolute and unconditional under any and all
      circumstances.

      There shall be no obligation of the Holders to give notice to, or obtain
the consent of, the Guarantor with respect to the happening of any of the
foregoing.

      SECTION 5.4 Rights of Holders.

      The Guarantor expressly acknowledges that: (a) this Guarantee Agreement
will be deposited with the Guarantee Trustee to be held for the benefit of the
Holders; (b) the Guarantee Trustee has the right to enforce this Guarantee
Agreement on behalf of the Holders; (c) the Holders of a Majority in Liquidation
Amount of the Preferred Securities have the right to direct the time, method and
place of conducting any proceeding for any remedy available to the Guarantee
Trustee in respect of this Guarantee Agreement or exercising any trust or power
conferred upon the Guarantee Trustee under this Guarantee Agreement; and (d) any
Holder may institute a legal proceeding directly against the Guarantor to
enforce its rights under this Guarantee Agreement, without first instituting a
legal proceeding against the Guarantee Trustee, the Issuer or any other Person.

      SECTION 5.5 Guarantee of Payment.

      This Guarantee Agreement creates a guarantee of payment and not of
collection. This Guarantee Agreement will not be discharged except by payment of
the Guarantee Payments in full (without duplication of amounts theretofore paid
by the Issuer) or upon distribution of Notes to Holders as provided in the Trust
Agreement.

      SECTION 5.6 Subrogation.

      The Guarantor shall be subrogated to all (if any) rights of the Holders
against the Issuer in respect of any amounts paid to the Holders by the
Guarantor under this Guarantee Agreement and shall have the right to waive
payment by the Issuer pursuant to Section 5.1; provided, that, the Guarantor
shall not (except to the extent required by mandatory provisions of law) be
entitled to enforce or exercise any rights it may acquire by way of subrogation
or any indemnity, reimbursement or other agreement, in all cases as a result of
payment under this Guarantee Agreement, if, at the time of any such payment, any
amounts are due and unpaid under this Guarantee Agreement. If any amount shall
be paid to the Guarantor in violation of the preceding sentence, the Guarantor
agrees to hold such amount in trust for the Holders and to pay over such amount
to the Holders in accordance with the terms of this Guarantee Agreement.

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<PAGE>

      SECTION 5.7 Independent Obligations.

      The Guarantor acknowledges that its obligations hereunder are independent
of the obligations of the Issuer with respect to the Preferred Securities and
that the Guarantor shall be liable as principal and as debtor hereunder to make
Guarantee Payments pursuant to the terms of this Guarantee Agreement
notwithstanding the occurrence of any event referred to in subsections (a)
through (g), inclusive, of Section 5.3.

      SECTION 5.8 Enforcement.

      A Beneficiary may enforce the Obligations of the Guarantor contained in
Section 5.1(b) directly against the Guarantor, and the Guarantor waives any
right or remedy to require that any action be brought against the Issuer or any
other person or entity before proceeding against the Guarantor.

                                   ARTICLE VI

                           COVENANTS AND SUBORDINATION

      SECTION 6.1 Dividends, Distributions and Payments.

      So long as any Preferred Securities remain outstanding, if there shall
have occurred and be continuing an Event of Default or the Guarantor shall have
entered into an Extension Period as provided for in the Indenture and such
period, or any extension thereof, shall have commenced and be continuing, then
the Guarantor may not (a) declare or pay any dividends or distributions on, or
redeem, purchase, acquire or make liquidation payment with respect to, any of
the Guarantor's capital stock or (b) make any payment of principal of or any
interest or premium on or repay, repurchase or redeem any debt securities of the
Guarantor that rank pari passu in all respects with or junior in interest to the
Preferred Securities (other than (i) repurchases, redemptions or other
acquisitions of shares of capital stock of the Guarantor in connection with any
employment contract, benefit plan or other similar arrangement with or for the
benefit of any one or more employees, officers, directors or consultants, in
connection with a dividend reinvestment or stockholder stock purchase plan or in
connection with the issuance of capital stock of the Guarantor (or securities
convertible into or exercisable for such capital stock) as consideration in an
acquisition transaction entered into prior to the occurrence of such Event of
Default or the applicable Extension Period, (ii) as a result of an exchange or
conversion of any class or series of the Guarantor's capital stock (or any
capital stock of a subsidiary of the Guarantor) for any class or series of the
Guarantor's capital stock or any class of series of the Guarantor's indebtedness
for any class or series of the Guarantor's capital stock, (iii) the purchase of
fractional interests in shares of the Guarantor's capital stock pursuant to the
conversions or exchange provisions of such capital stock or the security being
converted or exchanged, (iv) any declaration of a

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<PAGE>

dividend in connection with any rights plan, the issuance of rights, stock or
other property under any rights plan or the redemption or repurchase of rights
pursuant thereto, or (v) any dividend in the form of stock, warrants, options or
other rights where the dividend stock or the stock issuable upon exercise of
such warrants, options or other rights is the same stock as that on which the
dividend is being paid or ranks pari passu with or junior to such stock).

      SECTION 6.2 Subordination.

      The obligations of the Guarantor under this Guarantee Agreement will
constitute unsecured obligations of the Guarantor and will rank subordinate and
junior in right of payment to all Senior Debt of the Guarantor.

      SECTION 6.3 Pari Passu Guarantees.

            (a)   The obligations of the Guarantor under this Guarantee
      Agreement shall rank pari passu with the obligations of the Guarantor
      under any similar guarantee agreements issued by the Guarantor with
      respect to preferred securities (if any) similar to the Preferred
      Securities, issued by trusts other than the Issuer established or to be
      established by the Guarantor (if any), in each case similar to the Issuer,
      including, without limitation, the Guarantee Agreement, dated September
      17, 1999, issued by the Guarantor with respect to the preferred securities
      issued by MBWM Capital Trust I.

            (b)   The right of the Guarantor to participate in any distribution
      of assets of any of its subsidiaries upon any such subsidiary's
      liquidation or reorganization or otherwise is subject to the prior claims
      of creditors of that subsidiary, except to the extent the Guarantor may
      itself be recognized as a creditor of that subsidiary. Accordingly, the
      Guarantor's obligations under this Guarantee will be effectively
      subordinated to all existing and future liabilities of the Guarantor's
      subsidiaries, and claimants should look only to the assets of the
      Guarantor for payments thereunder. This Guarantee does not limit the
      incurrence or issuance of other secured or unsecured debt of the
      Guarantor, including Senior Debt of the Guarantor, under any indenture or
      agreement that the Guarantor may enter into in the future or otherwise.

                                   ARTICLE VII

                                   TERMINATION

      SECTION 7.1 Termination.

      This Guarantee Agreement shall terminate and be of no further force and
effect upon (a) full payment of the Redemption Price of all Preferred
Securities, (b) the distribution of Notes to the Holders in exchange for all of
the Preferred Securities or (c) full payment of the amounts payable in
accordance with the Trust Agreement upon liquidation of the Issuer.
Notwithstanding the foregoing, this Guarantee Agreement will

                                       16
<PAGE>

continue to be effective or will be reinstated, as the case may be, if at any
time any Holder must restore payment of any sums paid with respect to Preferred
Securities or this Guarantee Agreement. The obligations of the Guarantor under
Sections 3.3 and 3.4 shall survive any such termination or the resignation and
removal of the Guarantee Trustee.

                                  ARTICLE VIII

                                  MISCELLANEOUS

      SECTION 8.1 Successors and Assigns.

      All guarantees and agreements contained in this Guarantee Agreement shall
bind the successors, assigns, receivers, trustees and representatives of the
Guarantor and shall inure to the benefit of the Holders of the Preferred
Securities then outstanding. Except in connection with a consolidation, merger
or sale involving the Guarantor that is permitted under Article VIII of the
Indenture and pursuant to which the successor or assignee agrees in writing to
perform the Guarantor's obligations hereunder, the Guarantor shall not assign
its rights or delegate its obligations hereunder without the prior approval of
the Holders of a Majority in Liquidation Amount of all of the Preferred
Securities, voting together as a single class.

      SECTION 8.2 Amendments.

      Except with respect to any changes that do not adversely affect the rights
of the Holders in any material respect (in which case no consent of the Holders
will be required), this Guarantee Agreement may only be amended with the prior
approval of the Guarantor, the Guarantee Trustee and the Holders of not less
than a Majority in Liquidation Amount of all of the Preferred Securities, voting
together as a single class. The provisions of Article VI of the Trust Agreement
concerning meetings or consents of the Holders shall apply to the giving of such
approval.

      SECTION 8.3 Notices.

      Any notice, request or other communication required or permitted to be
given hereunder shall be in writing, duly signed by the party giving such
notice, and delivered, telecopied or mailed by first class mail as follows:

            (a)   if given to the Guarantor, to the address or facsimile number
      set forth below or such other address, facsimile number or to the
      attention of such other Person as the Guarantor may give notice to the
      Guarantee Trustee and the Holders:

                  Mercantile Bank Corporation
                  5650 Byron Center Avenue SW
                  Wyoming, Michigan  49509
                  Facsimile No.: (616) 406-3701
                  Attention: Chief Executive Officer

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<PAGE>

            (b)   if given to the Issuer, at the Issuer's address or facsimile
      number set forth below or such other address, facsimile number or to the
      attention of such other Person as the Issuer may give notice to the
      Guarantee Trustee and the Holders:

                  Mercantile Bank Capital Trust I
                  c/o Mercantile Bank Corporation
                  5650 Byron Center Avenue SW
                  Wyoming, Michigan  49509
                  Facsimile No.: (616) 406-3701
                  Attention: Administrative Trustee

            (c)   if given to the Guarantee Trustee, at the address or facsimile
      number set forth below or such other address, facsimile number or to the
      attention of such other Person as the Guarantee Trustee may give notice to
      the Guarantor and the Holders:

                  Wilmington Trust Company
                  Rodney Square North
                  1100 North Market St.
                  Wilmington, Delaware 19890-1600
                  Facsimile No.: (302) 636-4140
                  Attention: Corporate Trust Administration

            (d)   if given to any Holder, at the address set forth on the books
      and records of the Issuer.

      All notices hereunder shall be deemed to have been given when received in
person, telecopied with receipt confirmed, or mailed by first class mail,
postage prepaid, except that if a notice or other document is refused delivery
or cannot be delivered because of a changed address of which no notice was
given, such notice or other document shall be deemed to have been delivered on
the date of such refusal or inability to deliver.

      SECTION 8.4 Benefit.

      This Guarantee Agreement is solely for the benefit of the Holders and is
not separately transferable from the Preferred Securities.

      SECTION 8.5 Governing Law.

      THIS GUARANTEE AGREEMENT AND THE RIGHTS AND OBLIGATIONS OF EACH PARTY
HERETO, SHALL BE CONSTRUED AND ENFORCED IN ACCORDANCE WITH AND GOVERNED BY THE
LAWS OF THE STATE OF NEW YORK WITHOUT REFERENCE TO ITS CONFLICT OF LAWS
PROVISIONS (OTHER THAN SECTION 5-1401 OF THE GENERAL OBLIGATIONS LAW).

      SECTION 8.6 Submission to Jurisdiction.

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<PAGE>

      ANY LEGAL ACTION OR PROCEEDING BY OR AGAINST ANY PARTY HERETO OR WITH
RESPECT TO OR ARISING OUT OF THIS GUARANTEE AGREEMENT MAY BE BROUGHT IN OR
REMOVED TO THE COURTS OF THE STATE OF NEW YORK, IN AND FOR THE COUNTY OF NEW
YORK, OR OF THE UNITED STATES OF AMERICA FOR THE SOUTHERN DISTRICT OF NEW YORK
(IN EACH CASE SITTING IN THE BOROUGH OF MANHATTAN). BY EXECUTION AND DELIVERY OF
THIS GUARANTEE AGREEMENT, EACH PARTY ACCEPTS, FOR ITSELF AND IN RESPECT OF ITS
PROPERTY, GENERALLY AND UNCONDITIONALLY, THE JURISDICTION OF THE AFORESAID
COURTS (AND COURTS OF APPEALS THEREFROM) FOR LEGAL PROCEEDINGS ARISING OUT OF OR
IN CONNECTION WITH THIS GUARANTEE AGREEMENT.

      SECTION 8.7 Counterparts.

      This instrument 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.

                      [THE NEXT PAGE IS THE SIGNATURE PAGE]

                                       19
<PAGE>

      IN WITNESS WHEREOF, the undersigned have executed this Guarantee Agreement
as of the date first above written.

                                    MERCANTILE BANK CORPORATION

                                    By:   /S/ GERALD R. JOHNSON, JR
                                        ----------------------------------------
                                        Gerald R. Johnson, Jr.
                                        Chairman and Chief Executive Officer

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

                                    By:   /S/ W. THOMAS MORRIS, II
                                        ----------------------------------------
                                        Name: W. Thomas Morris, II
                                        Title: Senior Financial Services Officer


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