IMO, the length of the tank (for tangs) is really more important than the volume. Example: I think a 6', 125 gal tank is better for tangs (and most fish) than a 5' 150 gal. An 8' tank is even better. Tangs are not only quite large, for captive fish, they are very active. They live on the outer edges of the reef and really cover some distance. A 4 foot tank just doesn't give any tang (IMO) enough swimming room and this is bound to increase stress levels. Also IMO, a grown tang just looks out of place pacing back and forth in a 4' tank. I kept an Achilles for many years before moving and have had one for about 4 years now. I can't imagine these magnificent fish in a 4' tank.
Or, get a big circular tank and the fish can swim in nonstop circles. (Better add: just kidding)