Some Malawi cichlids, known as "mbuna," like lots of rocks in their tanks. Mbuna is a local name meaning "rock-dwelling cichlid." However, not all Malawi cichilds are mbuna.
Probably the most peaceful Malawi cichlids are the open-water dwellers, known in Lake Malawi as "utaka." Utaka do not defend territory, so they have less reason for aggression. However, your four-foot tank is really too small for any of the open-water species.
The smallest Malawi cichlids are probably the shell-dwellers, some of which are two inches long or so when adult. (There are also shell-dwellers from Lake Tanganyika and from other places, but they aren't Malawi cichlids, obviously.)
The most colorful perhaps are the "peacock cichlid" types (genus Aulonocara and genus Protomelas). They are not mbuna, but you could keep one male and several females in a four-foot tank (only the males are brightly-colored).
"Peaceful" and "colorful" tend to be contradictory terms in the fish world, and especially in the cichlid world. Having bright colors serves as a warning and a challenge to other fish, especially other members of the same species and sex. It's sort of like wearing gang colors in the human world: Don't do it unless you're willing to fight to defend your turf. So the brightly-colored fish tend to be the rough and tough ones.
As a first Malawi cichlid, you might like the Yellow Lab, Labidochromis Caeruleus. Both sexes are brightly colored, even as juveniles. This fish does like a lot of rock crevices for shelter. It is fairly aggressive, but no more so than most Malawi cichlids, and it stays small enough that you could have a colony of them in a four-foot tank.
1.50 pounds per fish is quite cheap for African cichlids. If your local shop is selling them for that price, I suspect that the shopkeeper doesn't know what species they are, either. They could also be hybrids (cross-bred from different species, usually by accident). I don't recommend getting a hybrid, although a few of them are becoming popular in the hobby.