what, praytell is the revision of the G4? If it's the 1st revision (867,900,1Ghz) then it's most likely the actual GPU, since apple didn't switch the position of the GPU when they switched from the G3 (which is the reason why the beginning G4's had the same problem as the G3s)

My laptop get slightly warm at max on the HDD area, but not *too* warm. Even with the 5400RPM 120GB HDD in it. Runs just fine and cool compared to when I had my G3 that had the GPU there