Personally, I find the best system to be a cycle system, but you need to have competent teammates (or at least communication), and that doesn't always happen.
Basically depending where the ball is, you stay back far enough that you can react to a big shot being sent your way, but also close enough that you have a chance at going in for a ball and getting it.
If you do happen to go in for a ball, then another teammate rotates back and takes your position.
Doesn't matter if you're playing 2v2, 3v3, or 4v4; that just means you'd have more players in the rotation.
But unfortunately most of the time you'll have a bunch of idiot teammates that BOOST UP TO WHERE THE BALL IS GOING TO FALL and then stop because wasting boost wasn't needed, and then they watch (or perhaps hop jump straight up in the air five feet) as an opponent who understands the logistics of timing and momentum sails over his head and crushes the ball djfasd;ghj;dksghjdskghsdjk