Does the G4 have Firewire? If it does the easiest way is to connect them with a Firewire cable and restart the G4 while holding down the T key. Then your G4 will show up as a hard drive in the iMac and you can just drag them over.

Do what @johnpowell says! I’ve done that at least three times, it works great. Anything else will be much more complex – and likely to be slower, too.