To my knowledge, the answer to both of your questions is yes.
If you don't mind playing around a bit you can always do this to be on the safe side:
1) Backup your Palm folder on (burn it to cd, copy to idisk, whatever)
2) Install Palm 4.0 for OS X
3) Run it. If it works, great, if not, restore your old version from your backup (or restore it from the OS X version by moving your home/documents/palm/users folder back to your OS 9 Palm folder.
Pending on the data you have on the device, I'd say give it a whirl. One things worth noting though, if you use AvantGo, prepare to kiss that goodbye under OS X. That's my big problem with OS X.
O
B unce!