From my experience, then, you could have a nice sized group. Most people don't think of keeping adult severums in groups, but ime it can be done as long as the tank is large enough for the number and size of fish. Unless you had something like two compatible pairs, I would do more than 4. Basically, ime keeping severums in a group is similar to other cichlids you can keep in groups, 5 or 6 tends to be a minimum group number to keep aggression low. When I had 8 in a 135 there was almost no aggression, even when a pair spawned they'd pick out a corner and go about their business but no real fighting.
Of course, severums can vary in temperament, so it's possible to get a mean one now and then, but what I've found is when you give them a large enough tank most severums are good citizens. I once took home a wild rotkeil that was killing anything they tried to keep with it-- they had it in something like a 25 gal tank-- took him home and put him in a 75 and he was never a problem for me, not even with angelfish and tall bodied tetras.