dragonfly,
I had both a Gen 4 and Gen 3 and I can honestly say that I just like the Gen 3 more. I have large hands (XL in most gloves) so the larger grip works for me. Honestly, I just shoot Glocks very well, not sure why. That being said, I hated how the initial draw stroke would often place the front sight high for me (1911s, CZs, and my Stingray-C seem to present perfectly) so I think the angle was a bit odd for me. What I did was use a gripforce adapter on my G19. Really helped a lot.
Most people I have spoken to say they really like the Gen 4's ergos. It is smaller without a backstrap and has more options, including a factory beavertail attachment. Word of advice, make sure the back of the Gen4 has the notch for the beavertail to attach to or it will be loose. The newest production models have it and you should make sure your Gen4 does to maximize the beavertail affect.