4" yellow tang-2 years old

Judging by your aqua-scape he will have plenty of room. As for him getting bigger, I want to see what smart people say. I have no idea of the growth rate of specific marine fish, all I know is my fish never reach the maximum size LiveAquaria tells me they will get lol.

 

you should be fine for a while with that size. I ahve kept up to 3 tanks in my 75 which is the same length as your 55. all were small specimens and did very well together. the tank was an old established one so no bioload issues.

as for this guy, they can get bigger. maybe 6-8" or so. I pulled a purple out of a guys tank that was 5" long and it has already grown in mine. they are very similar fish.