The Gen 5 incorporates some serious upgrades:
1. Takedown spring, slide catch spring, and trigger springs are less prone to breakage due to redesigned spring type and leverage on said springs.
2. Ambidextrous slide catch (that actually works for lefties.)
3. Finger grooves removed (lots of people hated them.)
4. Widened magazine well opening.
5. Cutout in the front of the magazine well to allow for ripping out a magazine.
6. New magazines have bright orange follower and extended floorplates.
7. Trigger pull feel is improved (slightly)
8. Target barrel is more accurate than previous generations.
9. Locking block pin deleted from design (no 40 caliber versions of Gen 5s are likely to be forthcoming, I would guess)
If I have the option I will always pick a Gen 5 moving forward.