That's the mac version of the blue screen of death. Usually a hardware problem.

Try reseating the RAM. Maybe the RAM chips became loose. While the power is off (battery removed) pull out the RAM stick and put it back in. If there is an airport card pull that out and put it back in. These are just general instructions, I'm not familiar with a G4 iBook and it's internals (if there are RAM chips that can be removed, etc.)
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