Really? I've got a 9 year old yellow tang who has been in a 120 his entire life and he's started pacing. I got him from a friend a couple of years ago who couldn't keep him due to downsizing (needed a good home and I was the only candidate at the time) and while he's perfectly healthy the 4' length is OBVIOUSLY not enough for him. Since he went from one 120 to another 120, I figured it would be ok...it's not. It's only a matter of time before the confines of the 4' tank really gets to them, in my opinion. I've got another option for a very large tank to put him in and that's where he's going...he's one of the very few lucky ones.
Not trying to be tang police really...just saying that I don't think any Zebrosoma tangs are really appropriate for 4' tanks, ultimately. Admittedly, I do fall onto the "no tang is suitable for any of our tanks" side of the fence a little bit though.