have you manually downloaded the proper xorg.conf for your G4 yet?
See this listing and choose your exact machine.
It'll provide you the xorg.conf you'll need to use the GUI.
You should start/restart linux in "single user mode" by typing
that's a 'number one' not the lowercase letter 'L'.
And once you get into single user mode, proceed to use the 'wget' command to download the proper xorg.conf for your machine. Then use the "mv" command to move the downloaded .TXT file into the /etc/X11 folder and rename it to "xorg.conf"
So, if I had a computer that needed to use the xorg file "imac7.txt" it'd look like this...
sudo mv imac7.txt /etc/X11/xorg.conf
The second line moves the file into the X11 folder and renames the file at the same time. Note that on some occasions, you don't even need to use 'sudo'.... but if the command fails without it, just use it.
While you're at it, you should also add yourself to the sudoers list, this will allow you to properly administer your computer once you're finally able to get into the GUI. See here:
When you're done, type "reboot", and you should be presented with a working GUI.