Measurments of the gen 4 vs the gen 3 measured from the back of the trigger guard to the curve of the backstrap.
Gen 3 = 2.242"
Gen 4 with no added backstrap = 2.154
Gen 4 with medium "M" backstrap = 2.245
Gen 4 with large "L" backstrap = 2.320
These measurements are for comparison only and are not intended to be any sort of official specs. I made them with a set of dial calipers from midway so although I was careful to be as accurate as possible there is some room for error.
I also measured the width of the grips and the Gen 3 was 1.185" wide while the Gen 4 was thinner at 1.178"
I'm sure there are many more measurements that people will be interested in, but the main point is the Gen 4 grip is shorter front to back and thinner side to side than the Gen 3 grip.