My Gen3 vs Gen4 experience, and yes I am a Glock enthusiast (or fan boy for all you haters);
I started with all 9mm Gen3s (26, 17, 19 & 34), and had no intention of changing. I had no complaints, and wondered why they were bringing out the Gen4. Like others have said, I think the sales loss to M&P was the biggest motivation. Once ejecting problems were solved with new springs, the Gen4 showed it is probably as reliable as the Gen3, but more time is needed to back that up. I was curious so I bought a used 17g4. I shot it, and my 17g3 equally for about 6 months. The G3 had some trigger work done to it before I bought it, so not a perfect comparison, but close enough. I felt they both had very similar recoil response, and reacted almost identically. The 3 was maybe a little snappier on recoil, but it was very slight, if at all.
Although they are almost identical, there are slight differences;
The grip size - Same as G3 when using the small backstrap, but 4 other options for different hand size. (+)
The grip texture - Definitely more aggressive, for a better hold. (+)
Ambi-mag release - Option for lefties or second finger mag release. (+)
Bigger mag release - Personally that's a plus for me, but variable depending on individual. (+/-)
Longer lasting, lower felt recoil new spring assembly - as long as it is reliable, the longer life span is a plus. Less recoil? Jury is out on that still, but would be nice if noticeable. (+)
Front Heavy - I never noticed any difference, but am going to check them both sometime today and see how they compare. STAY TUNED!
Overall I think the Gen4 has some very small improvements, even if they were not needed by most, they are still improvements overall. Since I bought the used G17G4, I bought a new G17G4FDE, sold my G17G3 (waiting fr CFO) and the G17G4 is on the EE. I sold my G19G3 then bought G19G4, sold my G34G3 and have a new G34G4MOS on order. My G26G3 I am not upgrading at this time.
I definitely prefer the new texture, bigger mag release and smaller grip option. I run all my Glocks with factory internal parts. The only changes I make are to Meprolight night sights, Glock extended mag release and Glock extended slide lock.