Shao,
Full grown these guys get to 40cm in length.
Mine grew to almost 5" long in less than a year. By the time it was that size it was flying around my tank at high speed. Yes it would graze but it still spent most of it's time moving. It would reach the glass and literally vibrate as if it still needed to go further.
GregT on RC that sent me a phenomenal pic of a school of blue tangs in the wild. Take a look at the size of the full grown angel mixed in with the Tangs. That will give you an indication of how big these fish are full grown.
IME a 4' tank won't be enough for this fish. These are one of the tangs that need at least an 8' tank. And even then. I won't do it.
http://www.fishbase.org/Summary/Spec...name=coeruleus