The temperature reporting features of the G4 were never accurate to begin with, and were dropped sometime after the release of the AGP G4s. I've got a PCI G4, which does support CPU temperature reporting, but the temperatures reported had a margin of error of 20 - 30 degrees F -- hardly a useful or accurate number.
Your machine probably does not support this feature. One good thing about the Mac is that you'll experience a sudden shutdown if your Mac gets too hot -- kind of a failsafe way of making sure you don't overheat.