Agreed will get small for rays; not quite sure what you mean by hard hitting fish; mean cichlids perhaps?
4x2 is a decent size for mid-sized central American cichlids, a couple which are pretty badass and could live in here would be Nansopsis tetracanthus, or Istlanum. A pair would be good, just make sure to provide the female a way to escape. Keeping an extra female or two in the tank might be smart as well. I had my pair of Panamensis cichlids in a 4x1.5 tank, and the male ended up killing the female, even though they were the only fish in the tank, and the male was only 5" long. The pair was ok previously with some extra non-paired individuals in the tank. I believe this can be helpful, as well as either a divider or flowerpot with a female-sized hole that she can escape through, that the male is too big to get into