There are many different philosophies on keeping pets.
You can cram whatever you want in your tank, but don't necessarily delude yourself that everything is healthy and in the best conditions. We have a natural tendancy to do this. Look on the board for how many threads are on something being KILLED rather than it dying. We hope to believe it was happy and killed, rather than the thought it died for some reason.
One thing I especially see in tanks is that the fish may not grow...or they may outgrow the tank and no one realizes it. Fish do not grow to their tank size so if it is not outgrowing the tank - there is a problem and they are not healthy. So therefor you want them to outgrow the tank, so you will have to trade them in or get a larger tank. If neither of those are options, you should reconsider your fish choice.
The counterpoint to the tang police argument is "its not the ocean anyway" and that is equally unproductive. there is a middle ground. Kole tangs and their relatives are fantastic, IMO, for that tank size and very much underrated. They are "rock huggers" as opposed to open water swimmers.
If anyone comes to ask, I intend to tell them my ideas and opinions. You can do whatever you want with that, but you'll still hear it, regardless of what you want to hear. I assume that is the point of asking. IMO, a 4' tank is not suitable for 3 tangs, long term, regardless of how many people have done it successfully.
I do ask people to go to their local aquarium where there is a big tank with tangs in it - often hippos, unicorns, nasos, sailfins...see how big they are, how much ground they cover in a single direction before turning...and then make a decision. You may still be fine with putting them in your 4' tank, its up to you. But do be open to doing something different, and do understand that you will have to do something eventually because the fish SHOULD outgrow the tank.
