Ok, some good suggestions here...but please stop suggesting Mbu puffers or Fahaka puffers for a 4' tank!!! This is waaay too small for a Mbu (mine outgrew his 3' in a few months, I had it shipped to me at 3"!), and I think a 5-6' tank is better suited for a Fahaka. So no puffers in there 
Sorry I can't suggest anything else that's predatory, active and that would fit in a small tank like this! I do like the suggestion of a quiet predator, with a small school of tinfoil barbs. I keep them myself, small school of 4, with my larger predators. They are very attractive, VERY active, and bullet proof (very hardy fish). They do get big with time tho, mines now are about 8" long each...so you'll need a new home for them long term.
And I can appreciate questioning like yours...nothing beats planification when stocking a tank!
Sorry I can't suggest anything else that's predatory, active and that would fit in a small tank like this! I do like the suggestion of a quiet predator, with a small school of tinfoil barbs. I keep them myself, small school of 4, with my larger predators. They are very attractive, VERY active, and bullet proof (very hardy fish). They do get big with time tho, mines now are about 8" long each...so you'll need a new home for them long term.
And I can appreciate questioning like yours...nothing beats planification when stocking a tank!
