The 3" has a 3" flush valve
The 4 has a 3" flush valve with a funnel on top.
Both tanks have a 3" outlet.
See how that works?
TOTO came out with class four.
So Kohler came out with class five.
The Eljer Titan leapfrogged to class seven
And then Kohler came out with class six to fil out the slots.
Toto had a 3" flapper, so American Standard came out with one, but Koher had class five 3-1/4", so American Standard put of a" flapper on top of it's 3" flush valve.
Kohler didn't really like it's 3-1/4" flapper, so now it's using a 2" +/- flush valve with a cannister. That's the new class five.
Confused yet?
TOTO also calls theirs, G-Max, Power Gravity, E-Max, all with 3" flush valves. And then of course there is Dual-Max.
But wait...........then you look at trapway design too. Nobody is naming them yet. Though TOTO has what the call a double siphon on thiers.
Some of the benifits are real, and some are marketing. You have to weed through it to find your preference. The newer bowls are all improved over the old stuff.