I know you're looking for an alternative OS, but the iBook G4's can run Tiger quite well. If you have one of the later models (1.33GHz or 1.42GHz), you'll have no problems running Leopard. Some people like MintPPC, which is a Debian Linux flavor for PowerPC Macs, but I don't have any experience with it. To be honest, there's not really any options that are worth using on a G4 because all the G4's are fully capable of running Tiger quite smoothly (so long as you max your RAM and optimize your OS).
What's the specific model/specs of your iBook G4?
Leopard disks (retail installs that is) can cost you your first born child. It amazes me how expensive it still is compared to SL!
After my last post I decided to give yellow dog Linux a shot on my powermac g4 digital audio (733mhz G4 - 512mb ram - 32mb nvidia) I was even able to get java re 6 running a play some slooooooow runescape.
...smokes Yellow Dog, which is unsupported these days.