Here's a fun way to practice your singles game with four (or more) people on the court: ping-pong doubles. In ping-pong, doubles players must alternate shots; so, after the first player on the team hits a ball, the other ball must be struck by her partner or the team loses a point.
Players can follow the same rules on a tennis court. This game is not only fun but also hard-work (because players must hit and get out of the way - i.e., be constantly in motion). In addition, if the opposing team consists of players with unique qualities (e.g., lefty, touch player, good mover, etc.) one can practice playing points against various types of players without getting accustomed to any particular style. So, if one point is played against a right-handed player with a double-handed backhand, the next point can be played against his partner who may be a lefty with a one-handed backhand (and so on). This stresses the brain a lot more in that you will have to constantly adjust for the situation at hand.
Another variation of this game allows a team to win 2 points (or more) for every point that the team wins inside of the service line. This is a great way to practice for doubles because the team learns how the two players think and construct points. That is, the team works on building doubles chemistry through singles play. This game also works with more than 4 players on the court and the rallies can be quite elaborate and lengthy.