You can't. The G4 powerbook runs a PowerPC CPU and Knoppix is compiled for x86, specifically 486 and up. You will need a live CD designed to run on the powerpc architechture. Fedora Core 5 has a rescue CD for powerpc at http://redhat.secsup.org/fedora/core/5/ppc/iso/ 106MB.
And Ubuntu 5.10 has a powerpc livecd at http://releases.ubuntu.com/5.10/ubun...ve-powerpc.iso which I found here http://releases.ubuntu.com/5.10/