Yeah...4 goldfish in a 5 gallon tank sounds like it's probably too many. Goldfish need their water changed alot more frequently because of the high amounts of amonia they give off. I take it you have a good filter? A feeder fish would probably help you greatly, but you really don't want to add another fish with them, plus, the waste put off by the goldfish would probably kill it. My advice would be to get another tank then look into a feeder fish or go to a specialty store and get a live plant that does well with goldfish...some actually feed and eliminate the waste put out.
