An iBook G4 has a PowerPC G4 processor. Linux Mint is only compatible with x86 and x86-64 processors, and not with PowerPC processors. If you want to run Linux on your iBook G4, there are other distros that you could use.
You could use Debian, for which their wiki has some help: http://www.debian.org/ports/powerpc/inst/pmac
For other PowerPC compatible distros, check this list: http://penguinppc.org/about-2/distributions/
. Note that the iBook G4 is classified as a "New World Mac" and has a 32 bit processor.
Edit: Note that MintPPC has nothing to do with Linux Mint, except for causing confusion by having a similar name.