It happens in the game of bridge, and it happens in the game of doubles in tennis. One player is late, keeping the other three waiting. But in tennis there are ways to put that waiting time to good use. You can play what's sometimes called Canadian doubles. One player takes on the other two. He plays the doubles lines while they have to keep the ball within the singles court. You can rotate positions every few games. Or you can organize a two-on-one drill.
The single player can hit shots cross-court alternately to the other two players who, in turn, always return the balls to him down the line. Or the two players can hit lobs while the single player hits back overheads.
Whatever you do when a fourth player is late for a doubles match, don't just stand around. Put the time to good use.