With four in the tank (little ones like the ones at petsmart) they will bond to who ever they choose. They will actually choose their mate. I mean you will get a pair at least but the other two might not be m/f so then they will not pair up. The pair will take over the tank and attack the other two. But since you have the 55 the you can just put the othe two which did not pair up in the 55. Or if you get two pairs then nevermind what I just said, lol.