I have a yellow and a red sea sailfin in my 90, they're doing great. I have only 65-70 pounds of rock, so they have a fair amount of room to swim. Sure 6ft is better, but so is 8, or 10, etc.
Uncrowded 4ft, IMO, is fine for many species of tangs.
(ya, I know, Wayne is gonna yell at me
)