Wayne, I agree a 6 is better than a 4, and 8ft is even better. But realistically, I think we like to say 6 to ease our guilt a bit. I watch my tangs and they swim for 4ft, stop, and take a left turn. I've watched the same species in a 6ft, they swim 6ft, stop, and make a left turn. In the big picture, really not much of a difference. Would the extra 2 ft matter if you were adding multiples? Perhaps. And maybe because of my style of rockscaping, the tangs feel more comfortable in being able to swim between the rocks and all around the perimeter of the tank. I always use considerably less than the recommended amount of rock, so the fish have quite a bit of room, even if it's not linear.
One day I hope to have an 8ft tank, then we don't need to discuss this
And at that time, if I notice a difference, I'll come back here and eat my words.