I would recommend reading over this answer I wrote. It sounds like you have a slightly different situation, but the problem is likely very similar, and I suspect the solution would be the same.

In short, I suggest you clean out your computer, reseat the heatsink with some arctic silver and go from there.

If you use our ID-Your-Mac tool to find out what kind of G4 you have, I can likely give you an appropriate guide to open up your computer too.