10.4 should install without any hacks on a G4 iBook, so I'm guessing the machine doesn't have enough RAM. I'd recommend at least 512MB for 10.4, or 1GB+ if you can possibly manage it.
Either it's that, or you've got a machine specific OS X disk... if that's the case then the method MacMan suggested should work.
DOH, where is my mind today ( in the trash apparently ), i should have realized that a G4 iBook was supported in tiger, maybe something is up with the optical drive, or Ram that could be the reason of tiger not installing on it.
oh BTW does a G4 iBook have any available firmware updates
http://docs.info.apple.com/article.html?artnum=86117
it list's a few iBooks as having firmware updates available, but i dont know if your model is one of the ones shown to have a firmware update for.
i have seen OS X incompatibilities on a supported machine that is running a older firmware and even unsupported machines, and updating the firmware fixed the issue.
one of the issues was using a modded version of the Leopard installer disk to install on unsupported machine ( a dual 500mhz G4 Power Mac). he didn't realize that it was running a old version of the firmware and Leopard would Kernel panic as soon as it would start to load. he finally found out and updated the firmware and it installed without a hitch.
but if your iBook dont have a firmware download to update to, then there is some other issue at hand.
like Tom said you should not have to use xpostfacto to install Tiger on that cause it should be a Tiger supported machine, or you have a machine specific OS X install CD/DVD and that can cause problems booting the installer or just failing to run at all. i would check to see if you need to update your firmware and then if it fails, go the xpostfacto 4 route.