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.
       
      
               
                     
                              crwdns2934105:0crwdne2934105:0
                     
      
                      crwdns2934113:0crwdne2934113:0
                   crwdns2915270:0crwdne2915270:0
      
   
   
      crwdns2889612:0crwdne2889612:0
            2