Severums get pretty big, but Rotkeil Severums are a couple inches smaller and are probably a better fit in a 4' long tank. As far as compatibility it depends on the fish. I have a young Rotkeil and an Angel sharing my 75 with tetras and Celebes Rainbowfish. The Rotkeil is the tank boss and occasionally chases the Angel but completely ignores the other fish. The Angel chases the other fish when he's bottom feeding (he's weird though) but doesn't mess with the Rotkeil.