Add a fourth major interest area, the performing arts, to Nancy Reagan's more publicized concerns of alcohol and drug abuse among youth, Foster Grandparents and White House redecorating.

It was she, not the president, who opened the first concert in the Public Broadcasting System series of musical performances at the White House and it is she, not the president, shown with Beverly Sills in television spots promoting the series.

Mrs. Reagan's image makers can partly thank Muffie Brandon, the increasingly influential White House social secretary. Not until Brandon came up with the idea of featuring young artists with established ones could anyone think of a way to keep the Sunday concert series from looking as if the Reagans were copying the Carters.

Despite one published report to the contrary, Brandon is more secure than ever in her job. She has won the support of Very Important Presidential aides in the West Wing. And among Mrs. Reagan's East Wing staff, with whom the first lady meets weekly, Brandon is the valued resident expert on Establishment Washington. Through her guidance, Mrs. Reagan has become a "regular" on the Georgetown-Kalorama private luncheon circuit that includes Evangeline Bruce, Ethel Garrett, Susan Mary Alsop, Oatsie Charles, Betty Beale and Selwa Roosevelt, among others.

Brandon's celebrity status among her children got a boost a couple of Sundays ago when "Doonesbury" satirized her and something it called "Foster Grandpreppies." Brandon's only complaint, she told comic strip artist Garry Trudeau in a fan letter, was that he hadn't spelled her name right.