What is the best set up for an aquarium?

Since my 8 year old goldfish is on his last fins, I will soon be redoing my tank. I have heard that tetras (mollys, neons, etc) are easier to take care of. Even with one goldfish, the tank gets dirty quickly (have filter and don't feed too much)
Also, it is a 4 gallon tank. I know larger is better, but it is my parents house and I am limited on space. (They don't want a large tank even though I take care of it).
Any ideas, or links to sites would be great.
Thanks!
The gold fish is about 2.5-3 inches and stopped growing several years ago.