They could vastly improve the PSP's online, 1 GB of ram, adobe flash 10 support, blue tooth, improved screen over 3000, and a more power efficient battery.
However, if they make the system's screen smaller, even by a centimeter, I will not buy it.. I'm still using a 2000 model, and I'm sure that if the 4000 model had everything the 2000 model did, + more, I could upgrade. I use my PSP a lot, although mostly for music and movies, not games, and I think it's time I sat this one in the drawer.
I want the UMD drive, so that I'm able to play my UMD games. If I want digital games, I can have them on my memory stick, and get the best of both worlds.