IME schooling fish will sometimes break into smaller groups and then join back into one large school again. I've noticed it several times in my own tanks, but it doesn't happen a lot. Except for a couple of rare exceptions, all my schools have 7 or more fish of the same specie.
Four fish is, IMO, not a school. You really need to have at least six for a school and more than that is better. IME that's why one of your fish ends up alone. When my schools break up there are never any fish left alone. They always break up in groups of three or more and there are never any pairs, always three is the smallest group.
Once in a while a fish finds himself alone, but you can see by his attitude that it's a mistake. He looks for his buddies ASAP and joins up with them.
Roan