yes they may have done so but dun you think its cruel to squeeze so many tangs into a small tank..... have you thought abt that...... yes 4 or 5 acanthurus may do well at on time but once territory has been established fight may occur and that can happen at any time any momement..... it can result in a killing frenzy whereby one tang or 2 just gang up and wack others to death then finally you are left with one..........
FYI in the wild PBT dun live in pair, they school thus a pair will definately fight and wack each other despit adding at the same time..... Trust me i have been there done that when I am still a newbie starting out...... I have seen many lfs placing PBT together, yes some may seem ok but do you notice on some tanks where PBT are placed together, Some of the PBT may have big slashes........ Okok you may want to disagree again but dun you think squeezing tangs together in a small tank is an ugly sight... Tangs are only beautiful when the are given ample of space.....
I will not want to repeat history but you can try 2 PBT together and update on them from the day you buy till they die and see if they really fight like i mention.... You cannot just base on the fact that you seen some reefer with tangs doing ok together and you say that 4 feet tank is ok with many tangs... Do you visit those reefer everyday from the day of addition till the death of the fish....
Hope you understand, I am not picking a fight... cos physically i surely lose