If you cram 2 4" bgks into an enclosure not much bigger than a one foot cube, you are asking for fights. I would return both of them unless you can provide a much larger tank right now.
once they get about 2 inches they start to hate each other. one way we get around it is put a ton in the tank to spread aggression (i dont reccommend that to u) but sooner or later they get moved to individual tanks. once they get about 3 inches they end up by themselves (and somtimes sooner).