Both are similar. Small hands like the stock Gen4 better than the 3. In my hands, short fingers like sausages. I can touch the thumb and middle finger while gripping the gen 4. The Gen3 leaves me with a 1/8th" gap between thumb and middle finger while gripped. That equates to a gun less rotated in my hand and more in line with arm for the Gen 4. Feels much nicer even though just a couple mm circumference difference.
As for parts, Visual inspection lends me to see the mag release is bigger and farther rearward. Grip and adapters. Texture on grips. Mags with extra release detent for ambi setup. Could be more, but those were the obvious without comparing part numbers.