pumpkinseeds are on the smaller end of the centrarchidae, and 4 in a 75 would be fine. Do you happen to know what sexes you have on hand? You should have either all male or one male and three females, but not a pair of pairs. That could lead to disaster should they get in the mood for spawning. My pumkinseed enjoys life in a 55 with an eel, a white cat, and a channel cat. Both the white and channel are getting moved to an indoor pond later this week, as they have outgrown the 55. The pumkinseed is definately the king of the tank though, and he's the smallest in there. Just watch for the agression.
Very personable fish. Mine stares at me the entire time I'm in my bedroom. Kinda weird having a fish stare at you in your, um, most private moments. He eats out of my hand, and is one of our favorite fish around here. He had a friend, but he killed him. With 4 fish of about the same size, you should be okay, and also if you introduce them all to the tank at the same time.
Pics please!