Target disk mode is probably your best bet. I have not tried it on a G4 Mac, but the functionality probably hasn't changed much. Here's the Apple knowledge base article about it:
http://support.apple.com/kb/ht1661
We use a Tableau T9 firewire write blocker when connecting a Mac using target disk mode. You'll need some form of firewire write blocking, of course.
The only hitch in this plan is if the Mac has an open firmware password set. If it does, it will fail to enter target disk mode when you hold down the T key and will instead boot the OS. To check for this, power on the Mac and immediately hold the option key. If the startup manager appears, you can power off and perform the target disk mode acquisition. If it prompts you for a password, you're going to need to reset the open firmware password, which is possible on a G4. Google can tell you how.
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