FOURTH AMENDMENT TO LEASE

Exhibit 10.6.4

FOURTH AMENDMENT TO LEASE

This Fourth Amendment to Lease (“Amendment”) is made and entered into this 18th day of June, 2008, by and between Glenn Dale Business Center, L.L.C. , a Maryland limited liability company (“Landlord”), and TVI Corporation , a Maryland corporation (“Tenant”).

W I T N E S S E T H:

WHEREAS , Landlord and Tenant are parties to a Lease dated February 16, 1998, as amended by the First Amendment to Lease dated December 10, 2002, as amended by the Second Amendment to Lease dated January 16, 2004, and as amended by the Third Amendment to Lease dated March 29, 2004 (collectively the “Lease”), whereby Tenant leases from Landlord certain premises containing approximately one hundred eighteen thousand two hundred seventy-three (118,273) square feet, known as Suites 200 and 200b (the “Premises”), located at 7100 Holladay Tyler Road, Glenn Dale, Maryland (the “Building”) for a Term which expires on April 30, 2010 (the “Current Term”); and

WHEREAS, Landlord and Tenant wish to amend the Lease by extending the Lease Term by an additional five (5) years and to otherwise modify the Lease.

NOW THEREFORE, in consideration of the mutual covenants herein made, Landlord and Tenant hereby enter into this Amendment and state as follows:

RECITALS. The foregoing recitals are incorporated herein by this reference.

CAPITALIZED TERMS. Unless otherwise defined herein, all capitalized terms shall have the same meaning as they have been assigned in the Lease.

LEASE TERM. Notwithstanding any provision of the Lease to the contrary, the Lease Term shall be extended by an additional five (5) years (the “Extension Term”) following the expiration of the Current Term. The Extension Term shall commence on May 1, 2010 (the “Extension Term Commencement Date”) and shall expire on April 30, 2015 (the “Extension Term Expiration Date”). Notwithstanding any provision of the Lease to the contrary, the Tenant shall have no right to extend the Lease Term beyond April 30, 2015.

PREMISES. Tenant acknowledges that Tenant is currently in possession of the Premises and, except as provided for in Paragraph 7 of this Amendment, Tenant hereby accepts the Premises in its “as is” condition, subject, however, to Landlord’s ongoing repair and maintenance obligations as required under the Lease and the following corrective actions, all of which Tenant has notified Landlord in writing:

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MINIMUM RENT. Tenant shall continue to pay Minimum Rent in the amounts and upon the terms and conditions set forth in the Lease through February 28, 2009. Beginning on March 1, 2009 and continuing throughout the Extension Term, Tenant shall pay Minimum Rent in the amounts set forth in the following schedule:

ADDITIONAL RENT. For the remainder of the Current Term and throughout the Extension Term, Tenant shall continue to pay its proportionate share of Real Estate Taxes, Common Facilities Costs for the operation, maintenance and management of the Common Facilities, and Insurance pursuant to the Lease. In addition, Tenant shall continue to be responsible for the costs and expenses of utilities, trash removal and janitorial services provided to the Premises and in accordance with the Lease, as well as all other Additional Charges specified in the Lease.

IMPROVEMENTS . (a) Promptly following the full execution of this Amendment, Landlord shall, in a good and workmanlike manner, and in compliance with all applicable laws, regulations, rules, orders, codes and ordinances, complete improvements to a portion of the Premises containing approximately 30,000 square feet (the “Renovated Premises”) in accordance with the following:

1) Tenant will cooperate with Landlord’s architect to promptly finalize the working drawings and specifications of materials for the improvements that Tenant desires beyond the “as is” condition of the Premises (the “Improvements”) (which final working drawings will comply with the ADA, and will incorporate and be consistent with the Building standards). The preliminary working drawings and specifications will be completed and attached hereto and made a part hereof as Exhibit A . Said drawings and specifications, when completed, will be ready to submit for necessary permits. Such drawings and specifications must be approved by Landlord prior to the commencement of any work related to the Improvements, which approval shall not be unreasonably withheld, conditioned or delayed. As modified by any Landlord reasonably-required changes, the final working drawings will be the “Final Plans”. If Landlord fails to respond to Tenant within ten (10) business days of request for approval, then Landlord shall be automatically deemed to have approved the working drawings as the “Final Plans.” Tenant is solely responsible for determining whether or not it is a public accommodation and for compliance with ADA within the Premises. Landlord’s approval of the plans, specifications and working drawings for Tenant’s alterations shall create no responsibility or liability on the part of Landlord for their completeness, design sufficiency, or compliance with all laws, rules and regulations of governmental agencies or authorities.

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2) Landlord will be the construction manager for the construction of the Improvements in accordance with the Final Plans. For the purpose of billing, the cost of the Improvements will be Landlord’s cost of constructing the Improvements, which includes a construction management fee of five percent (5%) of the total design and construction costs for supervision and coordination thereof. The cost of constructing the Improvements is estimated to be $363,773.00 as set forth in the Project Budget set forth on Exhibit B attached hereto and made a part hereof (the “Improvements Cost Estimate”). In the event the cost of the Improvements exceeds the Improvement Cost Estimate, Landlord agrees to notify Tenant in writing prior to performing the work and Tenant shall have the right to approve in advance the cost overruns or modify the Final Plans to reduce or eliminate the cost overruns. Landlord agrees that Tenant may use Tenant’s contractor to erect the walls separating the Renovated Premises from the remainder of the Premises.

3) Subject to Landlord’s obligations set forth in the Lease and in this Amendment, Landlord’s obligations with respect to improvements and alterations of the Premises shall be limited to the Improvements set forth in the Final Plans.

(b) Landlord will provide Tenant with an allowance (the “Improvement Allowance”) of up to $309,209.00 to be used for Improvements to the Premises. The Improvement Allowance may be used to pay: (a) the construction management fee; (b) the architectural and engineering fees, if any, relating to the Improvements [including: (i) the architectural fees related to the preparation of all drawings, (including CAD drawings) plans and specifications (collectively, “plans”), and (ii) engineering fees related