I would recommend reading over [http://www.ifixit.com/Answers/View/21509/Trackpad+Refuses+to+Lift|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 [http://www.ifixit.com/Apple-Parts/Arctic-Silver-Thermal-Paste/IF179-010|arctic silver] and go from there.
If you use our [http://www.ifixit.com/Info/ID-your-Mac|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.