Just bought a 19 Gen4 yesterday, picking it up Friday (waiting period...sigh).
The shop had Gen 3 19's that were trade-ins, so I got to do a side-by-side comparison, which was really nice.
The Gen 3's were in great shape, with night sights and 1 mag, for $479.
The Gen4 was through the GSSF discount, blue label package for $450 with case, 3 mags, speedloader, brush, backstraps, etc - the usual Glock Gen4 package.
I noticed that the grip for the Gen 3 was bigger than the Gen 4, and I liked the smaller grip (or at least the option to have a smaller grip with the Gen4, then use the backstrap for the Gen3 size if I wanted).
I also liked the tactile feel of the texture on the Gen 4 vs. the Gen 3. My biggest hesitation with getting the Gen 4 is that the Gen 3 is so well used and tested.
I fully expect to be happy with the Gen 4, or if nothing else I'll contact Glock and they'll work with me to be happy - if it requires a different spring or whatever, so be it.
Overall I think I like the Gen 4 better, but wouldn't hesitate getting a Gen 3 as well - i might pick up a Gen 3 30SF sometime.
Once I get the 19 to the range, I'll know more about how I really feel - but quite excited to be getting my first Glock.