if they are a m/f combo, each pair I mean, it won't be enough and any breeding pairs that might come out of the 4, will NOT tolerate the others....if they are all the same sex, you could be ok for awhile
You'll know when it's time to do something. When you see them start to become aggressive toward each other, I'd suggest making some other arrangements for one pair. It will start off with chasing and posturing, and then progress to nipped fins, lip locking, etc. I'm sure you'll want to take care of things before you get to that point, lol.