OK, I would definitely go with Apistogrammas - the Cacutoides are one of my faorites - but some others are also very colorful. And they will stay under your 4 inch limit
here are a few of my Triple Red Cacutoides:
Also Checkerboard Cichlids are pretty interesting as well - but harder to find:
Oh, just a basic FYI - while most fish will eat your basic flake food, it really is better for their long term health to feed them a varied diet - and mix in some other foods. I would suggest frozen bloodworms (or if you don't want frozen, get the freeze-dried ones)
Superstein - ever tried putting some oak leaves in the tank with the checkerboards? In their natural environment they turn leaves over looking for food - it is quite something to watch!