I've managed to find a table by one manufacturer (Kaindl) that has some actual data.
Basically AC3 vs AC4 comes down to twice the resistance before IP is reached in a Taber testTaber test, which uses sandpaper basically. IP is defined as: "The IP is reached immediately damage over an area of at least 0.60 m2 is suffered by the printed design in two quadrants and over an area of max. 0.60 mm2 in a third quadrant." I suspect there's a typo in there and the first measure is also 0.6 mm2 not m2.
Also AC4 is more resistant to impacts as well, about 50% better; I'm still a little confused about the precise meaning of that IC data.