It depends on what you want to do. The faster the G4 the faster encoding will be. Are you planning on capturing the video on the G4 or just converting it to different formats after editing?
I only have one OSX editing system designed for the G4, and that is a Matrox RTMAC running Final Cut Pro 2 on a B&W G3 upgraded to a G4-450 CPU (RTMAC needs a G4). It can capture analog as well as work with DV video (pre HD video). There are also older OS 9 era hardware and software (Targa 2000 hardware comes to mind with Premiere). There are other cheap capture cards out there for the G4 if you want to test them out.
If you want to do HD video maybe a G4 is too old.
I only have one OSX editing system designed for the G4, and that is a Matrox RTMAC running Final Cut Pro 2 on a B&W G3 upgraded to a G4-450 CPU (RTMAC needs a G4). It can capture analog as well as work with DV video (pre HD video). There are also older OS 9 era hardware and software (Targa 2000 hardware comes to mind with Premiere). There are other cheap capture cards out there for the G4 if you want to test them out.
If you want to do HD video maybe a G4 is too old.