The option-key trick *should* work, but I'm wondering if you have a Yikes! (pci graphics) G4 instead of a Sawtooth (agp graphics) G4. I think the PCI graphics model used a G3 motherboard or chipset, and I'm not 100% sure if those ol' Blue and White models had the option-key trick. I'm 99% sure they did, but I can't remember right now.
You might want to try zapping your PRAM, then go select it from the startup disk control panel, and see if it works.
To do this, hold down command(applekey)-option-P-R all at the same time when the computer first turns on. Keep holding it down, you'll hear your Chime, but then it'll Chime again when the PRAM is cleared.
-vasu