My apple won't boot past the startup screen.

So if you insert the OS disk in the optical drive, you should be able to turn on the computer while holding down the C key. It sounds like you are saying that won't even work?

Do you get any kind of display? Do you hear the startup chime? Is it possible your LCD went out and you just can't see what's happening?

If you are getting some kind of display, but the computer won't boot, try to start removing components until the computer boots. Start with the memory. Then unplug the optical drive, the the hard drive. Anything to see if you can get the computer to respond at startup.

I've never worked in the IMac G4, but if there are ethernet cards, airport cards, anything like that might be able to be disconnected.

The idea is, remove all devices possible until the only thing left is the logic board. If you still can't get any response, then you might have a bad logic board.