Here is a link to the part. I will also include a link that allows you to select your model G4. Once selected go to Airport install for instructions on how to install. The 12" 1.33 MHZ and the 14" 1.41 MHZ use a different card--link provided. Make sure you order the correct one.
Remove wires from your life / 802.11g internal card — 50+ available at 29.95 each.
802.11g Airport wireless / Bluetooth 2.0 internal card — 4 available at 69.95 each.