It of course depends on the tank size. But if you have a big enough tank it might not be a problem. Also depends on the tang's personality that you have. For instance my 3 tangs are kinda aggressive so I wouldnt dare add another tang unless he was BIG. LOL
You can also re arrange your rock and put one in, this causes the purple to have to get a ":new" territory and gives the new tang a chance to go into the tank without any aggression from the current tang. It's recommended that you add tangs all at once to avoid those types of issues.
180g Reef - 60g Rimless Reef -90G FOWLR- 125g Malawi Cichlids- 40b REEF- 34g Fresh Planted-working on- 20L FOWLR- working on
I have a mix of many different saltwater fish amongst my tanks, but I love my Tangs most of all.