Raising the O's together gives you a better chance of dwindling down aggression when they mature, but there are no promises. Especially if they pair up, like the others are saying, they will destroy the other two O's. But I had two O's in a 150 and they were fine (note:they were not mated). They actually followed each other around. But I think for four O's you'd want a 180 gl +. Plus lots of filtration.