One common case of Jam not working on the internal CDRW of the G4, is when the internal drive is a LITE-ON LTR-1210C with Revision numver RAVD. Earlier versions of this same drive would work with Jam. You can find info on your CDRW or other devices, by checking the Apple System Profiler.
There are some other CDRW drives in G4's that are not compatible with Jam 2.6.2 . Check the compatibility listing at roxio.com