I have to agree...from experience with my own 55 (1st saltwater tank), tangs just aren't very happy in a 55 and generally, when they aren't happy, they aren't healthy and end up suffering and ultimately, unless an upgrade is very near, they will more than likely not survive (I am sure that there are some people out there that will say that they have had success but they are the exception rather than the rule).
I would recommend 2 clowns, a 6-line wrasse, a dwarf angel (unless you are going for a reef...many angels will nip at corals), a couple of cleaner shrimp, and maybe an urchin of some sort, a couple of porcelain/anemone crabs (many people say that these hide, but mine were always front and center, especially at cleaning time), if you nix the angel, maybe a few green chromis...you just have to stay with fish that will stay smaller...bigger fish like swimming room, and while it may seem that when they are little that 4 feet is plenty, you have to realize that they won't always be little (it initially sucks when you find out that you can't have the fish that you want, but you then get to get creative!!!)