What size 4 foot tank? Big difference between a 55 and 120.
Right now I have two setups, one with large fish (two 12"+ pike cichlids, some silver dollars) and one with smaller fish (mix of CA cichlids and livebearers), and the setup with the smaller fish is much more satisfying for me to watch. Being able to watch the interactions of the fish in their environment, and seeing the fish have lots of room to explore, set up territories, quarrel, etc. is very rewarding.
What I would do is a colony of cryptoheros species, like Dan suggested. You will be able to watch them form pairs, breed, show off to each other, bicker over food and territory. Very interesting. Add some livebearers and bottom dwellers, and you will have activity all over the tank.
If your tank is a 55 gallon you should go with C. nanoluteus, they stay the smallest and you could keep 6 or so in the tank. Might have problems trying to do the larger species in that size. If it is a 75, 90, 120, I would say you could probably do any cryptoheros. Beware of convicts, they are generally harder to keep in groups in my experience.