This is one of those issues: Yes it can be done. But just b/c you can, don't mean you should. This is not at all in the best interests of the animal. It sounds like you may have a small coop/run set up. I suggest that you leave your current flock at 3 instead of trying to add a single chick. If you absolutely must have a 4th bird, and your coop run is at least 16 s.f. in coop and 40 s.f. in run, I suggest that you get a hen. But even that, I strongly advise against. If you don't have room to add 2 birds, going by the recommended space guidelines, don't add any.