i personally prefer 4.5, as its in the middle.
it depends on what size feet hes got.
IF he's got size 6(UK) 7(US) he shold probably only get 4.0, cheaper and you dnt need that much room. but if hes got anything bigger id say get 4.5. try find someone that has a 4.5 deck and try it out, see what he pefers