I've always felt that what 4A has over 6A and 5A is depth. I think the top four in each class are good and it may vary from year to year and weight to weight which are the best. I think the difference is beyond the top four or maybe beyond the placers.
I think the fact that less than half (when you consider some opens) of the wrestlers in each weight class qualify in 5A and 6A hurts the depth of those classes at the state tourney.
Of course, I coach at a 4A school and that probably effects my objectivity.