Debian's website has a comprehensive installation document for powerpc.
It doesn't quite get to the point quick enough.
So..
Navigate to http://sunsite.cnlab-switch.ch/ftp/mirror/debian/dists/woody/main/disks-powerpc/current/powermac/ and download:
Place those files in your root directory. That is, put them at the base of your partition/volume. Don't put them in any subdirectory in other words. They should just be dumped out onto the floor of your hard disk.
Next, reboot the G4, and capture the boot process at the Open Firmware stage.
At the OF prompt, type boot hd:x,yaboot where x is the partition number of the partition whre you put the files. If you are not sure which partition this is, just try 9 through 12, until it works.
From here see the installation manual referred to above, for how to continue.