I believe the g4's have usb 2.0 so your speeds should be much faster. But before we go into usb is the external hard drive using firewire or usb? Either the speeds should be way faster.

If either do 3 things.

1. use different usb/fw ports on the g4 (if they have extra, I never used a g4)
2. use different wiring.
3. ... try the external on another machine to see if it's your system that's the problem.

Sorry I couldn't help more. I will stay updated on this thread because I would like to know what could help solve an issue like this as well.