When Apple moved over to Intel chips everything had to be reprogrammed to work on the new architecture. There was a transition period where Apple were putting in an extra software layer to emulate old PowerPC code to the new Intel CPUs, but a point will always come when you just complete that transition and stop supporting older tech.
I have an old G3 Ruby iMac that runs OSX 10.2 (Panther), a friend of mine has the same one in Graphite that runs 10.4 (Tiger). I THINK Tiger is the last version of OSX to support PowerPC-based hardware.
PowerMac is the name for the old PowerPC-based systems, but there's never been a version of OSX that's restricted to a specific type of Apple computer, like an iBook Edition.
Nonetheless I believe you're right in saying 10.4 is as recent as you can go.