You probably should have posted this in the digital audio video forum, but here is a quick answer:
You need to specify which G4 you're planing on using because most powermacs and powerbooks have line level sound input, not mic level, meaning that if your mic doesn't already have its own pre-amp, you will have to buy one.
At this point, the best way to get audio into your mac is a usb pre amp. DV forge sells a single channel pre amp for about $40, or you can get a 2 channel pre amp from m-audio for $150.