There are a variety of different "goldfish" type fish around, multi coloured ones, black etc. What about a collection of those, with a 4ft tank you can have 20+ fish in there (depending on size of the fish).

Get some sucker fish and snails etc for the tank cleanliness, also watch where you put the tank in terms of natural light, a girlfriend has a tank in a dark part of her loungeroom and she is cleaning it every few days because of the algae build up (and she has suckers in there).

Also make sure there is enough filtration and air.

I'd talk to the aquarium shop for the best options, also get some different types of weed.