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                                 EXHIBIT 10.8.4

                            FOURTH AMENDMENT TO LEASE

      THIS FOURTH AMENDMENT TO LEASE ("Fourth Amendment") is made this 17th day
of May, 2000 by and between TA/WESTERN, LLC, a Delaware limited liability
company as successor in interest to THE EQUITABLE LIFE ASSURANCE SOCIETY OF THE
UNITED STATES ("Landlord") and VISUAL NETWORKS, INC., a Delaware corporation
("Tenant").


                              W I T N E S S E T H:

      WHEREAS, The Equitable Life Assurance Society of the United States,
Landlord's immediate predecessor in interest, and Tenant entered into that
certain Lease dated December 12, 1996, as amended by that certain Lease
Amendment dated September 2, 1997 (the "First Amendment"), as further amended by
that certain Second Lease Amendment dated February 8, 1999 (the "Second
Amendment") and as further amended by that certain Third Lease Amendment dated
January 10, 2000 (collectively, the "Lease"), pursuant to which Tenant leased
that certain premises in the building located at 2092 Gaither Road, Rockville,
Maryland (the "Building"), said leased premises containing approximately Fifty
Three Thousand Nine Hundred Forty Five (53,945) rentable square feet and
designated as Suites 100, 110, 120, 150, 200 and 220 (the "Original Premises");
and

      WHEREAS, Landlord and Tenant desire to amend the Lease to provide for a
renewal option with respect to the Term of the Lease and to amend certain other
terms and conditions of the Lease as herein provided.

      NOW, THEREFORE, in consideration of the premises and mutual covenants and
agreements herein contained and for other good and valuable consideration, the
receipt and sufficiency of which are hereby acknowledged, Landlord and Tenant
hereby agree to the following:

      1. RECITALS. The recitals set forth above are incorporated herein by this
reference with the same force and effect as if fully set forth hereinafter.

      2. CAPITALIZED TERMS. Capitalized terms not otherwise defined herein shall
have the meaning ascribed to them in the Original Lease.

      3. NOTICES. The notice addresses for Landlord set forth in Section 30 of
the Lease is hereby amended as follows:

            LANDLORD:         TA/Western, LLC
                              c/o McShea & Company, Inc.
                              One Bank Street, Suite 300
                              Gaithersburg, Maryland  20878
                              Attention:  Laurie Craft

            WITH COPY TO:     TA Associates Realty
                              28 State Street
                              Boston, Massachusetts 02109
                              Attention: James P. Knowles

      4. OPTION TO RENEW. Section 49 of the Lease is hereby deleted and the
following Section 49 substituted in lieu thereof:


            a. Provided that Tenant is not in default beyond any applicable cure
period at the time of Tenant's exercise of the Option, Tenant shall have one (1)
five (5) year option to renew this Lease. In the event Tenant exercises the
Option pursuant to this Paragraph 4, Tenant must concurrently exercise its right
to renew the lease by and between Landlord and Tenant dated as of the date
hereof, as the same may hereafter be amended, for the premises located at 2096
Gaither Road, Rockville, Maryland 20850. Tenant shall provide to Landlord on a
date which is prior to the date that the option period would commence (if
exercised) by at least two hundred seventy (270) days and not more than three
hundred sixty five (365) days, a written notice of the exercise of the option to
extend the Lease for the additional option term, time being of the essence. Such
notice shall be given in accordance with Section 30 of the Lease as modified by
Paragraph 3 hereinabove. If notification of the exercise of this Option is not
so given and received, the Option granted under this Paragraph 4 shall
automatically expire. Base Rent applicable to the Premises for the Option Term
shall be equal to the Fair Market Rental, as defined hereinafter. All other
terms and conditions of the Lease shall remain the same.


            b. If the Tenant exercises the Option, the Landlord shall determine
the Fair Market Rental by using its good faith judgment. Landlord shall provide
Tenant with written notice of such amount within fifteen


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(15) days after Tenant exercises its Option. Tenant shall have fifteen (15) days
("Tenant's Review Period") after receipt of Landlord's notice of the new base
rent within which to accept such rental. In the event Tenant fails to accept in
writing such rental proposal by Landlord, then such proposal shall be deemed
rejected and Landlord and Tenant shall attempt to agree upon such Fair Market
Rental, using their best good faith efforts. If Landlord and Tenant fail to
reach agreement within fifteen (15) days following Tenant's Review Period
("Outside Agreement Date") then the parties shall each within ten (10) days
following the Outside Agreement Date appoint a real estate broker who shall be
licensed in the State of Maryland and who specializes in the field of commercial
office space leasing in the Rockville, Maryland market, has at least ten (10)
years of experience and is recognized within the field as being reputable and
ethical. If one party does not timely appoint a broker, then the broker
appointed by the other party shall promptly appoint a broker for such party.
Such two individuals shall each determine within ten (10) days after their
appointment such base rent. If such individuals do not agree on Fair Market
Rental, then the two individuals shall, within five (5) days, render separate
written reports of their determinations and together appoint a third similarly
qualified individual having the qualifications described above. If the two
brokers are unable to agree upon a third broker, the third broker shall be
appointed by the President of the Montgomery County Board of Realtors. In the
event the Montgomery County Board of Realtors is no longer in existence, the
third broker shall be appointed by the President of its successor organization.
If no successor organization is in existence, the third broker shall be
appointed by the Chief Judge of the Circuit Court of Montgomery County,
Maryland. The third individual shall within ten (10) days after his or her
appointment make a determination of such Fair Market Rental. The third
individual shall determine which of the determinations of the first two
individuals is closest to his own and the determination that is closest shall be
final and binding upon the parties, and such determination may be enforced in
any court of competent jurisdiction. Landlord and Tenant shall each bear the
cost of its broker and shall share equally the cost of the third broker. Upon
determination of the base rent payable pursuant to this Section, the parties
shall promptly execute an amendment to this Lease stating the rent so
determined.

            c. The term "Fair Market Rental" shall mean the annual amount per
rentable square foot that a willing, comparable renewal tenant would pay and a
willing, comparable landlord of a similar office building would accept at arm's
length for similar space, giving appropriate consideration to the following
matters: (i) annual rental rates per rentable square foot; (ii) the type of
escalation clauses (including, without limitation, operating expenses, real
estate taxes, and CPI) and the extent of liability under the escalation clauses
(i.e., whether determined on a "net lease" basis or by increases over a
particular base year or base dollar amount); (iii) rent abatement provisions
reflecting free rent and/or no rent during the lease term; (iv) length of lease
term; (v) size and location of premises being leased; and (vi) other generally
applicable terms and conditions of tenancy for similar space; provided, however,
Tenant shall not be entitled to any tenant improvement or refurbishment
allowance. The Fair Market Rental may also designate periodic rental increases,
a new Base Year and similar economic adjustments. The Fair Market Rental shall
be the Fair Market Rental in effect as of the beginning of the Option period,
even though the determination may be made in advance of that date, and the
parties may use recent trends in rental rates in determining the proper Fair
Market Rental as of the beginning of the Option period.

            d. The Option granted to Tenant in this Fourth Amendment, is
personal to the original Tenant and may be exercised only by the original Tenant
while occupying the entire Premises and may not be exercised or be assigned,
voluntarily or involuntarily, by or to any person or entity other than Tenant,
including, without limitation, any permitted transfer as defined in Section 11
of the Lease. The Option herein granted to Tenant is not assignable separate and
apart from this Fourth Amendment, nor may this Option be separated from this
Fourth Amendment in any manner, either by reservation or otherwise. If at any
time the Option is exercisable by Tenant, the Lease has been assigned, or a
sublease exists as to any portion of the Premises, the Option shall be deemed
null and void and neither Tenant nor any assignee or subtenant shall have the
right to exercise the Option

      5. REAFFIRMATION OF TERMS. All other terms, covenants and provisions of
the Original Lease are hereby confirmed and ratified and except as modified
herein, shall remain unchanged and in full force and effect.

      6. REPRESENTATIONS. Landlord and Tenant each hereby represent and warrant
to the other that it (i) is not in default of any of its obligations under the
Original Lease and that such Original Lease is valid, binding and enforceable in
accordance with its terms, (ii) it has full power and authority to execute and
perform this Fourth Amendment, and (iii) it has taken all action necessary to
authorize the execution and performance of this Fourth Amendment.

      7. COUNTERPART COPIES. This Fourth Amendment may be executed in two or
more counterpart copies, each of which shall be deemed to be an original and all
of which counterparts shall have the same force and effect as if the parties
hereto had executed a single copy of this Fourth Amendment.

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      IN WITNESS WHEREOF, Landlord and Tenant have executed this Fourth
Amendment as of the day and year first above written.

                              LANDLORD:

                              TA/Western, LLC, a Delaware limited liability
                              company




                              BY: REALTY ASSOCIATES ADVISORS LLC, A DELAWARE
                                       LIMITED LIABILITY COMPANY, MANAGER





                                     BY: REALTY ASSOCIATES ADVISORS TRUST, A
                                           MASSACHUSETTS BUSINESS TRUST, SOLE


                                                         MEMBER


                                          BY:   /S/   JAMES P. KNOWLES
                                                ------------------------------
                                                      JAMES P. KNOWLES
                                                      REGIONAL DIRECTOR


                              TENANT:

                              VISUAL NETWORKS, INC., a Delaware corporation

                              By:   /s/   PETER J. MINIHANE
                                    ------------------------------------------
                                          Peter J. Minihane
                                          Chief Operating Officer