It depends on which PowerMacintosh G4 model you're referring to -- there are quite a few.
Most that I know of either have a standard VGA port, or a DVI port with a DVI-to-VGA adaptor. Some also have an ADC port, but most that do have the ADC port also include a standard VGA or DVI port as well.
Do you have access to the G4 in question? If so, a quick peek behind will answer your question. If not, maybe you can post a link to the computer in question to be absolutely sure.