Personally I like Synodontis catfish, less predatory in general and a bit more visible. S. nigriventris, aka common upside down catfish, are fairly easy to find and probably the least expensive. They are a social, schooling fish, best kept in a group. As with any schooling fish, the more the merrier (more active/visible). The corydoras are also quite popular; they are also social critters, best kept in groups. All of these fish are smaller, none likely to exceed 8-9cm.
The larger species of Synodontis are less inclined to be social, but sevral can be kept in a 4' tank provided they each have a suitable territory. Properly cared for, they can live more than 20 years, though spawning is most unlikely (with a few exceptions).