Ron, since I don't see how I could complicate things more than they already are, have you tried the following?
Treat the G5 the same way you would an Airport base station (ie, router).
I think this involves hooking the G4 to the G5 with an ethernet cable (and maybe even a cross over cable at that) and running the Airport Admin utility, or whatever it's called.
Do that, shut down G4, disconnect cable, reboot G4. (Come to think of it, you might want to shut down and reboot the G5 while you're at it.)
Or heck, you may be able to do all that wirelessly nowadays, for all I know.
But I seem to remember that's how we set up our original Airport base station. Which has never given us a single pause in over 3-4 years of daily use.
It sounds as if there's some setting in your G5 which isn't being transferred over to the G4. It could be something as obscure as the Airport card's serial number which differs from the one in the G4.
Failing the above, try running your mouse over both thighs.
Good luck!