One must-win begat another must-win situation for the Rangers. For just as surely as the Rangers did not want to lose the first three games of their Eastern Conference quarterfinal to the Capitals, they must be desperate to avoid going back to Washington for Game 5 on the brink of elimination. Given an early emotional boost by defenseman Marc Staal’s return from a horrific facial injury and with their power-play finally providing some scoring, the Rangers beat the Capitals, 4-3, Monday night at Madison Square Garden. The Capitals lead the series, 2-1, with Game 4 Wednesday night at the Garden. Derek Stepan scored the winner at 13:35 of the third period, tipping in Rick Nash’s sharp-angle shot from the right. “I think we’re close,” said Rangers coach John Tortorella, calling the series “more even” than the scoreboard indicated. “It means squat if you don’t win.” And to win, the Rangers had to defend the final 1:54 with the Capitals on the power play after Brad Richards tapped Alex Ovechkin with his stick at center ice and Ovechkin sold it for a slashing call. That became a six-on-four advantage when Braden Holtby (26 saves) was pulled for an extra skater. Henrik Lundqvist stopped 28 shots.