The Gen 4 has more aggressive texture for a better grip, med and large backstraps (pic shows no backstrap added). Larger mag release.
Gen 3 on the right, Gen 4 on the left. The Gen 4 has a beefier locking block and a wider channel between the rails to accommodate the larger recoil spring assembly.
The Gen 3 on top has a single flat wire captive spring. The Gen 4 below has a dual round wire captive spring.
Slightly different extractor. Gen 3 on top.
The Gen 4 on the bottom has a slightly larger ejector.
The Gen 4 magazine has several small areas where the metal frame is exposed. The cartridge appears to ride slightly higher in the Gen 4. I've shot using Gen 3 magazines in the Gen 4 without any issues. Haven't tried it the other way around.
I'm sure there are more differences. Those are just the obvious ones. They both shoot the same and both have had zero malfunctions. I do however like the grip of the Gen 4 much better.