I posted this on Guitpic's thread, but it really should have been here...so:

It's like Korg took the PA3X, addressed everything I needed, and put it all the 4X!
My favorite new features:
1. Great new guitar sounds. Also lots of other new sounds, Saxes, Violins and more. Fantastic Piano.
2. Able to save Chord Sequencer to Songbook.
3. Big new adjustable tilt screen!
4. Ability to audition or play styles from USB, needed long ago.
5. New sound engine--everything sounds better, even styles from the 3x sound better.
6. It feels even lighter in weight than the 3x. Not sure, but it feels like it.
7. A number of key or touch strokes eliminated...such as highlighting a SB entry automatically loads it, no need to hit a Select button. (This did create a small problem. Where the Select box was, there is now an Add To List box. I have hit it several times out of habit, and it automatically adds my SB entry to a Set List. No big deal, I can easily delete them if I want. I have quickly learned not to touch that box unless I want to save it though!)
8. It remembers several settings on startup that the 3x did not...Autofill, Syncho Start, STS mode all come to mind.
9. It automatically saves a number of parameters once you select them, no need to do extra Save procedures.
10. I really like the new Performance setup, now called Set List. I've just scratched the surface of its capabilities.
11. Ability to copy and paste styles to whatever locations you want. 3x didn't have this; PA900 did.
12. A good number of new styles, including a lot I can actually use. Of course they took out some of my old favorites, but I've already loaded them back.
These are just the things that come to mind quickly. I've had several comments from regulars about how good it sounds, and I didn't tell them I'd made a change.