It could even be using Appletalk as a network protocol - you should be able to connect any monitor. If it is a recent G4 it may have a DVI connector - but the adaptors are readily available, every G4 came with one (for the squarer of the two). You will probably want to connect a keyboard and mouse - any USB keyboard will do, even a "Windows" keyboard where the "Windows Key" will become the "Apple Key". Configure the network, enable SSH access, and then you can do just about everything from teh command line.
I'm assuming it is Mac OS X