yep - I know Im a thicko, but just when or where does one:It'll run on a G4. I installed Fienix on my 17" powerbook not that long ago. It has issues with graphics, but can be worked around. I just don't remember everything i did. Anyway, here's what i remember.....
Boot in to single user mode: Linux single
This will bring you to the text based log in.
Log in and su or sudo to root user.
Get a network connection (ethernet required and may require manually starting dhcp or the like)
NOTE: all root commands will need to be done by typing the whole path in single user mode (e.g. /sbin/reboot)
Run sudo apt update
Install some other DE or WM (xfce4 or icewm are good choices) and also install another session manager as the default session manager (try maybe lightdm, xdm, etc) and Mate are causing the graphics issue.
Once those are installed reboot and graphical login should work. Don't use Mate. Choose your new DE or WM.
There may be more to it than that, but that's what i remember when i installed it. It should get you going in the right direction at least.
Cheers
'Boot in to single user mode: Linux single'
Im hitting the same black screen McLin found - on my iBook.