Do you want to use that PowerMac? or just wipe the drive?
Just for an OS X install, you could connect the PowerMac to another Mac with a Firewire cable, booting the old tower to Target Disk mode.
The other Mac would also need to be one capable of booting to the same version of OS X that you would be installing, and you would just use the old PowerMac as a destination for the install.
How do you intend to use the old G4 (just curious)?
Do you have an OS X installer old enough for that PowerMac?
Some will be limited to 10.4 - which will be fine on any G4, but 10.5 is a possibility.
You won't be able to do anything, other than wipe the drive, unless you have an acceptable OS X installer that will boot your PowerMac.
You may want to check for the old OS X installer, if the PowerMac was being trashed, then the software might already be gone.