We have on occasion tested the limits of Apple hardware and have always been amazed by its longevity. In keeping with Apple's latest Mac OS. Who would've imagined that a PowerMac G4 400MHz which was originally released on Apple's OS 9 platform, can still until today run not only Mac OS X 10.4 Tiger but also Mac OS X 10.5 Leopard!
To install Mac OS X 10.4 Tiger on a PowerMac G4 400MHz (AGP) you actually have to update the firmware on the Logicboard of the PowerMac first. The firmware 4.2.8 can be downloaded together with the instructions from Apple's support web page.
For most of us, we probably don't have an OS 9 OS to do this anymore. So an alternative that will also work is to install Mac OS X 10.4 Tiger onto the intended hard drive for the PowerMac G4 using an external hard drive casing and another Mac.
Once that is completed, simply plant the Mac OS X 10.4 Tiger installed hard drive into the PowerMac G4 and boot it up.
This workaround can also be applied to install Mac OS X 10.5 Leopard but the caveat is that it has to be done from a system that is certified to fulfil the minimum specs required to run Mac OS X 10.5 Leopard.