I work for a cabinetmaker, and $4k sounds very expensive for the size you've quoted - the only reason I can think of is that they're going to make stand alone cupboards to fit the space, which is the same as what we would do. Believe it or not, a lot of cabbies won't do built in stuff as its too fiddly, much easier to build a cabinet that they can set up in the factory to make sure it's alright before it goes out to you.

Maybe contact a builder local to you, and ask who they use for built in robes?