Document:

<PAGE>   1
                                                                Exhibit 10.16

                                 AMENDMENT #4

        To Lease dated October 9, 1992 between Welsh Edenvale Partners '86 (a
Minnesota limited partnership), as Lessor and Stratasys, Inc. (a Delaware
corporation), as Lessee.

        THIS AMENDMENT TO LEASE, entered into and made as of July 27, 1998, by
and between Carpenter Land Company, L.L.P., as Landlord and Stratasys, Inc. (a
Delaware corporation), as Lessee.

                             W I T N E S S E T H:

        WHEREAS, Lessor and Lesee have heretofore entered into a Lease Agreement
(hereinafter referred to as the "Lease"), dated October 9, 1992 for
office/warehouse space located at 14950 Martin Drive, Eden Prairie, Minnesota,
(hereinafter the "Demised Premises"), commencing November 1, 1992; and

        WHEREAS, Carpenter Land Company, Landlord, purchased the building
including the Demised Premises on April 30, 1993; and

        WHEREAS, Landlord and Lessee amended the Lease on January 23, 1995 in
Amendment #1; and

        WHEREAS, Landlord and Lessee amended the Lease on June 22, 1995 in
Amendment #2; and

        WHEREAS, Landlord and Lessee amended the Lease on October 30, 1995 in
Amendment #3.

        NOW, THEREFORE, in consideration of the rents reserved and of the
covenants and agreements herein set forth, it is agreed by Landlord and Lessee
that the Lease is hereby amended from and after the date hereof as follows:

1.  TERM
      The lease term shall be extend by thirty-six (36) months, commencing on
August 1, 1999 and terminating on July 31, 2002.

2.  BASE RENT
      The base rent for the extension period shall be as follows:

                                                Base Rent
Period                                          Per Month           Annual Rent
------                                          ---------           -----------
August 1, 1999 through and
including July 31, 2000                        $13,938.00           $167,256.00

August 1, 2000 through and
including July 31, 2001                        $14,519.00           $174,228.00

August 1, 2001 through and
including July 31, 2002                        $15,100.00           $181,200.00
                                                                    -----------

                                                TOTAL               $522,684.00

<PAGE>   2
Amendment #4
Page 2

3.  IMPROVEMENTS
      The Landlord agrees to provide, at no expense to Lessee, the following
      improvements to the Demised Premises:

      -    Re-paint the building exterior in a mutually acceptable color.
      -    Provide a $5,000.00 allowance to be used to construct a new, lighted
           building monument sign.
      -    Overlay the asphalt in the loading area on the east side of the
           building.
      -    Modify the HVAC systems by adding two (2) new air conditioning
           units, new ducting and rebalance air flow to correct temperature
           and control deficiencies.

      The Landlord agrees to begin work on these improvements as soon as
reasonably possible after this Amendment is fully executed.  Any improvements
other than those described above must be approved by the Landlord and shall be
completed at the sole cost of the Lessee.

      Except as herein above set forth, all terms, provisions and covenants
of the Lease shall remain unchanged and in full force and effect.

      IN WITNESS WHEREOF, the parties hereto have executed this Amendment #4 as
of the date and year first above written.

Lessee:                                           Landlord:
STRATASYS, INC.                                   CARPENTER LAND COMPANY L.L.P.
(a Delaware corporation)

By: /s/ D.W. Moffatt                          By: /s/ J. Carpenter
----------------------------                      ---------------------------

Its: Chief Operating Officer                  Its: General Partner
----------------------------                      ---------------------------

                                              By: /s/ A.C. Kelly
                                                  ---------------------------

                                              Its: General Partner
                                                  ---------------------------<PAGE>   1
                                                                     EXHIBIT 4.a

          Temporary Certificate - Exchangeable for Definitive Engraved
                      Certificate - When Ready for Delivery

                                 MASCOTECH, INC.
               4 1/2% Convertible Subordinated Debenture Due 2003

REGISTERED                                                 CUSIP No. 574670 AB 1
No. TR

MascoTech, Inc., a corporation duly organized and existing under the laws of the
State of Delaware (herein referred to as the "Company"), for value received,
hereby promises to pay to __________________ or registered assigns, at the
office or agency of the Company in the Borough of Manhattan, The City of New
York, the principal sum of ____________________________ Dollars on December 15,
2003, in such coin or currency of the United States of America as at the time of
payment shall be legal tender for the payment of public and private debts, and
to pay interest, semi-annually on June 15 and December 15 of each year, on said
principal sum at said office or agency, in like coin or currency, at the rate
per annum specified in the title of this Debenture, from the June 15 and
December 15, as the case may be, next preceding the date of this Debenture to
which interest has been paid or duly provided for, unless the date hereof is a
date to which interest has been paid or duly provided for, in which case from
the date of this Debenture, or unless no interest has been paid or duly provided
for on the Debentures since the original issue date (as defined in the Indenture
referred to on the reverse hereof) of this Debenture, in which case from January
21, 1994, until payment of said principal sum has been made or duly provided
for. Notwithstanding the foregoing, if the date hereof is after June 1 or
December 1, as the case may be, and before the following June 15 or December 15,
this Debenture shall bear interest from such June 15 or December 15; provided,
however, that if the Company shall default in the payment of interest on such
June 15 or December 15, then this Debenture shall bear interest from the next
preceding June 15 or December 15 to which interest has been paid or duly
provided for, or, if no interest has been paid or duly provided for on the
Debentures since the original issue date (as defined in such Indenture) of this
Debenture, from the January 21, 1994. The interest so payable on any June 15 or
December 15 will, subject to certain exceptions provided in such Indenture, be
paid to the person in whose name this Debenture is registered at the close of
business on the June 1 or December 1, as the case may be, next preceding such
June 15 or December 15, whether or not such June 1 or December 1 is a business
day, and may, at the option of the Company, be paid by check mailed to the
registered address of such person.

Reference is made to the further provisions of this Debenture set forth on the
reverse hereof. Such further provisions shall for all purposes have the same
effect as though fully set forth at this place.

This Debenture shall not be valid or become obligatory for any purpose until the
certificate of authentication hereon shall have been signed by or on behalf of
the Trustee under such Indenture.

<PAGE>   2

IN WITNESS WHEREOF, MascoTech, Inc. has caused this instrument to be executed in
its corporate name by the facsimile signature of its Chairman of the Board or
its President and imprinted with a facsimile of its corporate seal, attested by
the facsimile signature of its Secretary or an Assistant Secretary.

Dated:   ______________

MascoTech, Inc.

By: Richard Manoogian
   Chairman of the Board

Attest: Eugene A. Gargaro, Jr.
   Secretary

CERTIFICATE OF AUTHENTICATION

This is one of the securities of the series designated therein referred to in
the within-mentioned indenture.

MORGAN GUARANTY TRUST COMPANY OF NEW YORK,
                                    AS TRUSTEE

BY________________________
AUTHORIZED OFFICER

                                       2
<PAGE>   3

                              REVERSE OF DEBENTURES
                                 MASCOTECH, INC.
               4 1/2% CONVERTIBLE SUBORDINATED DEBENTURE DUE 2003

         This Debenture is one of a duly authorized issue of debentures, notes,
bonds or other evidences of indebtedness of the Company (hereinafter called the
"Securities") of the series hereinafter specified, all issued or to be issued
under and pursuant to an indenture dated as of November 1, 1986 (herein called
the "Indenture"), duly executed and delivered by the Company to Morgan Guaranty
Trust Company of New York, Trustee (herein called the "Trustee"), to which
Indenture and all indentures supplemental thereto reference is hereby made for a
description of the rights, limitations of rights, obligations, duties and
immunities thereunder of the Trustee, the Company and holders of the Securities.
The Securities may be issued in one or more series, which different series may
be issued in various aggregate principal amounts, may mature at different times,
may bear interest (if any) at different rates, may be subject to different
redemption provision (if any), may be subject to different sinking, purchase or
analogous funds (if any), may be subject to different covenants and Events of
Default and may otherwise vary as in the Indenture provided. This Debenture is
one of a series designated as the 4 1/2% Convertible Subordinated Debentures Due
2003 of the Company, limited in aggregate principal amount to $345,000,000.

         Subject to the provisions of the Indenture, the holder of this
Debenture is entitled, at such holder's option, at any time on or after March
22, 1994 and prior to December 15, 2003 (except that, in case this Debenture or
any portion hereof shall be called for redemption, such right shall terminate
with respect to this Debenture or portion hereof, as the case may be, so called
for redemption at the close of business on the last business day preceding the
date fixed for redemption as provided in the Indenture, unless the Company shall
default in the payment due upon redemption thereof), to convert the principal
amount of this Debenture (or any portion hereof which is $1,000 or an integral
multiple thereof), into shares of Common Stock of the Company (calculated to the
nearest 1/100th of a share), as said shares shall be constituted at the Date of
Conversion, at the Conversion Price of $31.00 principal amount of Debentures for
each share of Common Stock, or at the adjusted Conversion Price in effect at the
Date of Conversion determined as provided in the Indenture, upon surrender of
this Debenture, together with the conversion notice hereon duly executed, to the
Company at the designated office or agency of the Company in the Borough of
Manhattan, The City of New York, accompanied (if so required by the Company) by
instruments of transfer, in form satisfactory to the Company and to the Trustee,
duly executed by the holder or by such holder's duly authorized attorney in
writing. Such surrender shall, if made during any period beginning at the close
of business on a record date and ending at the opening of business on the
interest payment date next following such record date (unless this Debenture or
the portion being converted shall have been called for redemption on a
redemption date during such period) also be accompanied by payment in New York
Clearing House funds or other funds acceptable to the Company of any amount
equal to the interest payable on such interest payment date on the principal
amount of this Debenture then being surrendered for conversion. Except as
aforesaid no adjustment is to be made on conversion for interest accrued hereon
of for dividends on shares of Common Stock issued on conversion. The Company is
not required to issue fractional shares upon any such conversion, but shall make
adjustment therefor in cash on the basis of the market value of such fractional
interest as provided in the Indenture.

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<PAGE>   4

         The indebtedness evidenced by the Debentures is, to the extent and in
the manner provided in the Indenture, subordinate and subject in right of
payment to the prior payment in full of the principal of (and premium, if any)
and interest on all Senior Indebtedness as defined in the Indenture, and this
Debenture is issued subject to such provisions and each holder of this
Debenture, by accepting the same, agrees to and shall be bound by such
provisions, and authorizes the Trustee in such holder's behalf to take such
action as may be necessary or appropriate to effectuate as between the holders
of the Debentures and the holders of Senior Indebtedness the subordination as
provided in the Indenture and appoints the Trustee such holder's
attorney-in-fact for such purpose.

         In case an Event of Default with respect to the 4 1/2% Convertible
Subordinated Debentures Due 2003 shall have occurred and be continuing, the
principal hereof may be declared, and upon such declaration shall become, due
and payable, in the manner, with the effect and subject to the conditions
provided in the Indenture.

         The Indenture contains provisions permitting the Company and the
Trustee, with the consent of the holders of not less than 66 2/3% in aggregate
principal amount of the Securities at the time outstanding of all series to be
affected (voting as a class), evidenced as in the Indenture provided, to execute
supplemental indentures adding any provisions to or changing in any manner or
eliminating any of the provisions of the Indenture or of any supplemental
indenture or modifying in any manner the rights of the holders of the Securities
of each such series; provided, however, that no such supplemental indenture
shall (i) extend the final maturity of any Security, or reduce the rate or
extend the time of payment of interest thereon, or reduce the principal amount
thereof or any premium thereon, or reduce any amount payable on redemption
thereof, or make the principal thereof or any interest or premium thereon
payable in any coin or currency other than that hereinbefore provided, or impair
the right to convert the 4 1/2% Convertible Subordinated Debentures Due 2003
into Common Stock on the terms defined in the Indenture, or impair or affect the
right of any holder to institute suit for payment thereof or the right of
repayment, if any, at the option of the holder, or modify any of the provisions
of the Indenture relating to the subordination of the Securities in a manner
adverse to the holders thereof, without the consent of the holder of each
Security so affected, or (ii) reduce the aforesaid principal amount of
Securities of all series to be affected, the holders of which are required to
consent to any such supplemental indenture, without the consent of the holders
of all Securities so affected then outstanding. It is also provided in the
Indenture that, with respect to certain defaults or Events of Default regarding
the Securities of any series, prior to any declaration accelerating the maturity
of such Securities, the holders of a majority in aggregate principal amount of
the Securities of such series at the time outstanding (or, in the case of
certain defaults or Events of Default, all the Securities) may on behalf of the
holders of all of the Securities of such series (or all the Securities, as the
case may be) waive any such past default or Event of Default under the Indenture
and its consequences except a default in the payment of principal of, premium,
if any, or interest, if any, on any of the Securities. Any such consent or
waiver by the holder of this Debenture (unless revoked as provided in the
Indenture) shall be conclusive and binding upon such holder and upon all future
holders and owners of this Debenture and any Debentures which may be issued in
exchange or transfer hereof or in substitution herefor, irrespective of whether
or not any notation thereof is made upon this Debenture or such other
Debentures.

         No reference herein to the Indenture and no provision of this Debenture
or of the Indenture shall alter or impair the obligation of the Company, which
is absolute and unconditional, to pay the

                                       4

<PAGE>   5

principal of and interest on this Debenture at the place, at the respective
times, at the rate and in the coin or currency herein prescribed.

         The Debentures are issuable in registered form without coupons in
denominations of $1,000 and any integral multiple of $1,000. In the manner and
subject to the limitations provided in the Indenture, but without the payment of
any charge (except for any tax or other governmental charge imposed in
connection therewith), Debentures may be exchanged for an equal aggregate
principal amount of Debentures of other authorized denominations at the office
or agency of the Company for such exchange in the Borough of Manhattan, The City
of New York or at such other location or locations as may be provided for
pursuant to the Indenture.

         The Debentures may be redeemed at the option of the Company as a whole,
or from time to time in part, on any date on or after December 22, 1996 and
prior to maturity, upon mailing a notice of such redemption not less than thirty
nor more than sixty days prior to the date fixed for redemption to the holders
of Debentures at their last registered addresses, all as provided in the
Indenture, at the following optional redemption prices (expressed in percentages
of the principal amount to be redeemed) together in each case with accrued
interest to the date fixed for redemption; provided, however, that if the date
fixed for redemption of any Debenture is an interest payment date, then the
regular semi-annual payment of interest becoming due on such date shall be
payable to the registered holder of such Debenture at the close of business on
the applicable record date.

              If redeemed during the twelve-month period beginning
December 15.

<TABLE>
<CAPTION>

         Year                                                                   Percentage
         ----                                                                   ----------
<S>                                                                            <C>
         1996.....................................................................103.00%
         1997.....................................................................102.50%
         1998.....................................................................102.00%
         1999.....................................................................101.50%
         2000.....................................................................101.00%
         2001.....................................................................100.50%
         2002.....................................................................100.00%
</TABLE>

         Upon due presentment for registration of transfer of this Debenture at
the office or agency of the Company for such registration in the Borough of
Manhattan, The City of New York, or any other location or locations as may be
provided for pursuant to the Indenture, a new Debenture or Debentures of
authorized denominations for an equal aggregate principal amount will be issued
to the transferee in exchange therefor, subject to the limitations provided in
the Indenture, without charge except for any tax or other governmental charge
imposed in connection therewith.

         The Company, the Trustee and any agent of the Company or the Trustee
may deem and treat the holder hereof as the absolute owner of this Debenture
(whether or not this Debenture shall be overdue and notwithstanding any notation
of ownership or other writing hereon), for the purpose of receiving payment of
or on account of the principal hereof and, subject to the provisions on the face
hereof, interest hereon, and for all other purposes, and neither the Company nor
the Trustee nor any such agent shall be affected by any notice to the contrary.
All payments made to or upon the order

                                       5

<PAGE>   6

of such holder shall, to the extent of the sum or sums paid, effectually satisfy
and discharge liability for moneys payable on this Debenture.

         No recourse for the payment of the principal of, or premium, if any, or
interest on this Debenture, or for any claim based hereon or otherwise in
respect hereof, and no recourse under or upon any obligation, covenant or
agreement of the Company in the Indenture or any indenture supplemental thereto
or in any Debenture, or because of the creation of any indebtedness represented
thereby, shall be had against any incorporator, stockholder, officer or
director, as such, past, present or future, of the Company or of any successor
corporation, either directly or through the Company or any successor
corporation, 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 issue
hereof, expressly waived and released.

         All terms used in this Debenture which are defined in the Indenture
shall have the respective meanings ascribed to them therein.

         This Debenture shall be deemed to be a contract made under the laws of
the State of New York, and for all purposes shall be construed in accordance
with and governed by the laws of that State.

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

         The following abbreviations, when used in the inscription on the face
of this certificate, shall be construed as though they were written out in full
according to applicable laws or regulations:

         TEN COM - as tenants in common
         TEN ENT - as tenants by the entireties
         JT TEN  - as joint tenants with right of
                   survivorship and not as tenants
                   in common

         UNIF GIFT MIN ACT -                         Custodian
                             ------------------------         ------------------
                                    (Cust)                           (Minor)
                             under Uniform Gifts to Minors
                             Act
                                ---------------------
                                     (State)

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

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

                                       6
<PAGE>   7

I or we assign and transfer this Security to

PLEASE INSERT SOCIAL SECURITY OR OTHER
IDENTIFYING NUMBER OF ASSIGNEE
 ----------------------------------------------------
|                                                    |
|                                                    |
 ----------------------------------------------------

--------------------------------------------------------------------------------
             (Print or type name, address and zip code of assignee)

--------------------------------------------------------------------------------
and irrevocably appoint

--------------------------------------------------------------------------------
to transfer this Security on the books of the Company. The agent may substitute
another to act for him

Dated                                        Signed:
     --------------------------------               ----------------------------

--------------------------------------------------------------------------------
        (Sign exactly as name appears on the other side of this Security)

                                CONVERSION NOTICE

TO MASCOTECH, INC.:

         The undersigned owner of this Debenture hereby irrevocably exercises
the option to convert this Debenture into shares of Common Stock of the Company
in accordance with the terms of the indenture referred to in this Debenture, and
directs that the shares issuable and deliverable upon the conversion together
with any check in payment for fractional shares and any Debentures representing
any unconverted principal amount hereof, be issued and delivered to the
registered holder hereof unless a different name has been indicated below. If
shares are to be issued in the name of a person other than the undersigned, the
undersigned will pay all transfer taxes payable with respect hereto. Any amount
required to be paid by the undersigned on account of interest accompanies this
Debenture.

Dated:
      --------------------------------       -----------------------------------
                                                    Signature

Fill in for registration of shares of Common Stock and Debentures if to be
issued otherwise than to the registered holder.

--------------------------  Social Security or other Taxpayer Identifying Number
      (Name)

--------------------------  ----------------------------------------------------
      (Address)

---------------------------------------------------------
Please print name and address (including zip code number)

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