What strategies can you use as a Renegade in 4 player games to maintain your cover?
I'm not sure Bang! is really about maintaining cover, anyway. Even for big games, it seems like ideal Bang! strategy is...

1) Outlaws try to spike down the Sheriff roughly ASAP, though they can sometimes wait until most players have long-range guns before exposing themselves.

2) Once the Outlaws are dead, the Renegade immediately tries to kill the weakened Sheriff before he gets time to recover.

The only subtlety I see is that, if the Outlaws are having a tough time, the Renegade should stop shooting them.

You're right, though, that in 4-player, because the range is so short, Outlaws don't even need to wait until long-range guns are out. And there's no moment of wondering who the Renegade is -- in larger games, that moment is brief, but it does exist.