<PAGE>

                                                                     Exhibit 4.1

                      CERTIFICATE EVIDENCING CLASS B UNITS
                    REPRESENTING LIMITED PARTNER INTERESTS IN
                         NORTHERN BORDER PARTNERS, L.P.

NO. 1                                                   36,494,126 CLASS B UNITS

     The undersigned officer of Northern Border Partners, L.P., a Delaware
limited partnership (the "Partnership"), hereby certifies that
____________________ (the "Holder") is the registered owner of 36,494,126
Class B Units representing limited partner interests in the Partnership (the
"Class B Units") transferable on the books of the Partnership, in person or by
duly authorized attorney, upon surrender of this Certificate properly endorsed
and accompanied by a properly executed application for transfer of the Class B
Units represented by this Certificate. The rights, preferences and limitations
of the Class B Units are set forth in, and this Certificate and the Class B
Units represented hereby are issued and shall in all respects be subject to the
terms and provisions of, the Amended and Restated Agreement of Limited
Partnership, as amended by Amendment No. 1 to the Amended and Restated Agreement
of Limited Partnership, as further amended, supplemented or restated from time
to time (the "Partnership Agreement"). Copies of the Partnership Agreement are
on file at, and will be furnished without charge on delivery of written request
to the Partnership at, the principal office of the Partnership located at 13710
FNB Parkway, Omaha, Nebraska 68154. Capitalized terms used herein but not
defined shall have the meanings given them in the Partnership Agreement.

     The Holder, by accepting this Certificate, is deemed to have (i) requested
admission as, and agreed to become, a Limited Partner and to have agreed to
comply with and be bound by and to have executed the Partnership Agreement, (ii)
represented and warranted that the Holder has all right, power and authority
and, if an individual, the capacity, necessary to enter into the Partnership
Agreement, (iii) granted the powers of attorney provided for in the Partnership
Agreement and (iv) made the waivers and given the consents and approvals
contained in the Partnership Agreement.

THE UNITS REPRESENTED BY THIS CERTIFICATE HAVE NOT BEEN REGISTERED UNDER THE
SECURITIES ACT OF 1933, AS AMENDED, OR ANY STATE SECURITIES LAWS ("ACTS"). THE
UNITS HAVE BEEN ACQUIRED FOR INVESTMENT AND MAY NOT BE SOLD OR OFFERED FOR SALE
IN THE ABSENCE OF AN EFFECTIVE REGISTRATION STATEMENT FOR THE UNITS UNDER THE
ACTS OR AN OPINION OF COUNSEL SATISFACTORY TO THE PARTNERSHIP THAT SUCH
REGISTRATION IS NOT REQUIRED.

This Certificate shall not be valid for any purpose unless it has been
countersigned and registered by the Transfer Agent and Registrar.

Dated:
       --------------------------

                                        NORTHERN BORDER PARTNERS, L.P.


Countersigned and Registered by:        By:
                                            ------------------------------------
                                            Chief Financial and Accounting
                                            Officer


Northern Plains Natural Gas Company,
LLC,
as Transfer Agent and Registrar


By:
    ---------------------------------
    Authorized Signature

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[Reverse of Certificate]

                                  ABBREVIATIONS

The following abbreviations, when used in the inscription on the face of this
Certificate, shall be construed as follows according to applicable laws or
regulations:

<TABLE>
<S>         <C>                                  <C>
TEN COM--   as tenants in common                 UNIF GIFT/TRANSFERS MIN ACT
TEN ENT--   as tenants by the entireties                    Custodian
                                                 (Cust)                  (Minor)

JT TEN--    as joint tenants with right of       under Uniform Gifts/Transfers to CD Minors Act
            survivorship and not as tenants in   (State)
            common
</TABLE>

Additional abbreviations, though not in the above list, may also be used.

FOR VALUE RECEIVED, __________________________ hereby assigns, conveys, sells
and transfers unto

_______________________________          _______________________________________
(Please print or typewrite name          (Please insert Social Security or other
    and address of Assignee)                 identifying number of Assignee)

__________ Class B Units representing limited partner interests evidenced by
this Certificate, subject to the Partnership Agreement, and does hereby
irrevocably constitute and appoint ____________________ as its attorney-in-fact
with full power of substitution to transfer the same on the books of Northern
Border Partners, L.P.

Date:                                   Note: The signature to any endorsement
      -------------------------------         hereon must correspond with the
                                              name as written upon the face of
                                              this Certificate in every
                                              particular, without alteration,
                                              enlargement or change.

      THE SIGNATURE(S) MUST BE
      GUARANTEED BY AN ELIGIBLE
      GUARANTOR INSTITUTION (BANKS,           ----------------------------------
      STOCKBROKERS, SAVINGS AND LOAN          (Signature)
      ASSOCIATIONS AND CREDIT UNIONS
      WITH MEMBERSHIP IN AN APPROVED
      SIGNATURE GUARANTEE MEDALLION           ----------------------------------
      PROGRAM), PURSUANT TO S.E.C.            (Signature)
      RULE 17Ad-15

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

No transfer of the Class B Units evidenced hereby will be registered on the
books of the Partnership, unless the Certificate evidencing the Class B Units to
be transferred is surrendered for registration or transfer and an Application
for Transfer of Class B Units has been properly completed and executed by a
transferee either (a) on the form set forth below or (b) on a separate
application that the Partnership will furnish on request without charge. A
transferor of the Class B Units shall have no duty to the transferee with
respect to execution of the Application for Transfer of Class B Units in order
for such transferee to obtain registration of the transfer of the Class B Units.

<PAGE>

                    APPLICATION FOR TRANSFER OF CLASS B UNITS

     The undersigned ("Assignee") hereby applies for transfer to the name of the
Assignee of the Class B Units evidenced hereby.

     The Assignee (a) requests admission as a Substituted Limited Partner and
agrees to comply with and be bound by, and hereby executes, the Amended and
Restated Agreement of Limited Partnership of Northern Border Partners, L.P. (the
"Partnership"), as amended by Amendment No. 1 to the Amended and Restated
Agreement of Limited Partnership, as further amended, supplemented or restated
to the date hereof (the "Partnership Agreement"), (b) represents and warrants
that the Assignee has all right, power and authority and, if an individual, the
capacity necessary to enter into the Partnership Agreement, (c) gives the powers
of attorney provided for in the Partnership Agreement and (d) makes the waivers
and gives the consents and approvals contained in the Partnership Agreement.

     Capitalized terms not defined herein have the meanings assigned to such
terms in the Partnership Agreement.

Date:
      -------------------------------   ----------------------------------------
                                        Signature of Assignee


-------------------------------------   ----------------------------------------
Social Security or other                Name and Address of Assignee
identifying number of Assignee

-------------------------------------
            Purchase Price
    including commissions, if any

Type of Entity (check one)

[ ] Individual [ ] Partnership [ ] Corporation
[ ] Trust [ ] Other (specify)

Nationality (Check One):

[ ] U.S. Citizen, Resident or Domestic Entity

[ ] Foreign Corporation, or [ ] Non-resident alien

     If the U.S. Citizen, Resident or Domestic Entity box is checked, the
following certification must be completed.

     Under Section 1445(e) of the Internal Revenue Code of 1986, as amended (the
"CODE"), the Partnership must withhold tax with respect to certain transfers of
property if a holder of an interest in the Partnership is a foreign person. To
inform the Partnership that no withholding is required with respect to the
undersigned interest-holder's interest in it, the undersigned hereby certifies
the following (or, if applicable, certifies the following on behalf of the
interest-holder).

Complete Either A or B:

     A.   Individual Interest-Holder

          1.   I am not a non-resident alien for purposes of U.S. income
               taxation.

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          2.   My U.S. taxpayer identifying number (Social Security Number) is

               ______________________________________________________________.

          3.   My home address is _________________________________________.

     B.   Partnership, Corporate or Other Interest-Holder

          1.   _____________________________________________ is not a foreign
                         (Name of Interest-Holder)

          corporation, foreign partnership, foreign trust or foreign estate (as
          those terms are defined in the Code and Treasury Regulations).

          2.   The interest-holder's U.S. employer identification number is

               ______________________________________________________________.

          3.   The interest-holder's office address and place of incorporation
               (if applicable) is ____________________________.

     The interest-holder agrees to notify the Partnership within 60 days of the
date the interest-holder becomes a foreign person.

     The interest-holder understands that this certificate may be disclosed to
the Internal Revenue Service by the Partnership and that any false statement
contained herein could be punishable by fine, imprisonment or both.

     Under penalties of perjury, I declare that I have examined this
certification and to the best of my knowledge and belief it is true, correct and
complete and, if applicable, I further declare that I have authority to sign
this document on behalf of

         ---------------------------------------------------------------
                            (Name of Interest-Holder)


         ---------------------------------------------------------------
                               Signature and Date

         ---------------------------------------------------------------
                              Title (if applicable)

     Note: If the Assignee is a broker, dealer, bank, trust company, clearing
corporation, other nominee holder or an agent of any of the foregoing, and is
holding for the account of any other person, this application should be
completed by an officer thereof or, in the case of a broker or dealer, by a
registered representative who is a member of a registered national securities
exchange or a member of the National Association of Securities Dealers, Inc.,
or, in the case of any other nominee holder, a person performing a similar
function. If the Assignee is a broker, dealer, bank trust company, clearing
corporation, other nominee owner or an agent of any of the foregoing, the above
certification as to any Person for whom the Assignee will hold the Class B Units
shall be made to the best of the Assignee's knowledge.