The G5 is several generations ahead, is faster, quieter, and can run Classic if that's needed (though I much prefer period beige hardware or the black PBG3s for older Mac OS versions). For OSX, and the large range of PPC-only software that is available, the G4 is a poor competitor for a G5. Keep the G5.

I have noticed over the years that a lot of people on here seem to hate the G5s. I wonder how many of them have actually used a G5 on a regular basis. I have, and like them a lot.

If you have a G5 that works, keep it on your desk. They are capable, elegant machines.