If you own a copy of OS4 and have a copy of the SDK, the documentation states pretty clearly how the OS4 boot process works. The rest requires a little bit of knowledge of how Open Firmware works. Everyhing you need to know has been spelled out in this thread.
And at some point, I imagine the ops will kill this thread. I'm being a pessimist, I know, but I'd really like to see a legitimate bootloader for PowerPC Macs out there, and this isn't the way to go about it.