Well, a 400 Mhz G4 is no speed demon when it comes to OS X, but if my 400 Mhz G3 iMac can tackle it, then you shouldn't have much of a problem.
If you really need power, then you might want to look into upgrading. But if you don't need raw power, and this is working out just fine, then you should enjoy Panther on your Powerbook.