I like the Gen 4s myself, just because of the versatility of the back-straps. I love the fact that the Gen 4 backstraps have the beavertail build in. I have had slide-bite from high-gripping the Gen 3s. I still have a Gen 3 G19 and have no intention of "upgrading." It is a great pistol.
I carried a G23 and G19 concealed for a bunch of years. For me, at least, it is a bit thick and heavy for comfortable all day/everyday carry. It's possible, I did it, but have been MUCH happier with the Shield as a daily/go anywhere gun. My G19 is now my nightstand/truck gun.
"I wanna go fast" -Ricky Bobby
"Mind bottling isn't it?" -Chazz Michael Michaels