A Beltsville aluminum housing supply firm has been awarded more than $4.5 million in an antitrust verdict against Hunter Douglas Inc. -- a Durham, N.C., company -- and Whittaker Corp. of Los Angeles. The two firms were charged with monopolizing key facets of the business in the Washington area.

The verdict, which was issued by a jury last week in federal court in Baltimore, was a victory for RCM Supply Co. Inc., a company that before filing for bankruptcy last August was a manufacturer and seller of aluminum siding, gutters, shutters and other accessory equipment.

RCM, since 1971, had distributed Hunter equipment, in part because of a line of credit agreement between the two companies. In 1977, RCM began purchasing equipment from a competitor of the company, Revere Manufacturing Co.

But in 1979, Hunter, according to the suit, told RCM that the company would require greater payments on outstanding balances and later demanded payments at the time of delivery. Further, the suit, brought under both federal and state antitrust law, charged that Hunter "engaged in predatory pricing and unfairly solicited customers" of RCM.

The complex suit was brought on April 5, 1980, and the 4 1/2-week trial closed just before the jury's verdict last week. The case is expected to be appealed.

The company was founded in 1970 by Lawrence McCarthy and, as recently as last year, had sales of about $5 million, according to the company's attorney, Leonard Goldstein. The company business was about evenly split between distributing aluminum products and acting as an applicator, installing siding on homes for large builders.

Under the bankruptcy laws, the company has been putting together a reorganization plan for creditors. The creditors did not require a final plan while the suit was pending.