John McEnroe won 4 U.S. Open singles titles.
His victories came in 1979, 1980, 1981, and 1984. McEnroe defeated Vitas Gerulaitis in the 1979 final, Björn Borg in 1980 and 1981, and Ivan Lendl in 1984. His three consecutive championships from 1979 through 1981 made him the first man since the 1920s to win three straight U.S. Open singles titles.
The 1984 victory was his fourth U.S. Open championship and his seventh—and final—Grand Slam singles title. He defeated Lendl in straight sets, 6–3, 6–4, 6–1. McEnroe also reached the 1985 U.S. Open final, but lost to Lendl, so his total remained four wins.
A common source of confusion is McEnroe’s doubles record. He won additional U.S. Open doubles titles, including four with Peter Fleming, but the question asks specifically about singles. His seven major singles titles consisted of four U.S. Opens and three Wimbledons.