The first generations of the G4 tower (Blue fronts) did not support an Airport card. Not until the 350 AGP models and the 'Quick Silver' models (Silver fronts) have the Airport socket and offered as optional install.
In all cases the original Airport is all you can install (802.11b version). These are getting hard to find locally. Here's an eBay listing eBay Airport cards. Be careful make sure you can get your money back if it doesn't work.
Depending on your OS you might find a 3rd party USB plugin WiFi option (drivers could be a problem). In this case you might find a 802.11n device.
Or, you could use an AirPort Express, AirPort Extreme or Time Capsule which have a WiFi connection (802.11n and newer) you can then connect over via an Ethernet connection which I think is your best option (other wired systems can then share the WiFi connection). Later you can use it as your own AP.