First off, are the 4 of reproductive age yet? If not, you'll just have to wait to see. When they are older and in a smaller tank, you'll see two hanging close to each other and, eventually, each of the other two turning more black (stress colors) and hanging out in the corners. You might also see some nipping or chasing. As soon as you see two hanging out together, remove the others.
Good luck! Right now I have five males (in separate tanks) and one female. But, I found a few captive-bred ones that are coming in next week so I can try to pair my extras. Fortunately, I have a certain male (he's carried eggs) that I can use to sex the newcomers when they are old enough.