USB adapters are generally not that great, unless you have USB 2. Apple did not introduce USB 2 until September 2003 on Macs. Try to identify your PowerMac G4 here:
And that will tell us what Airport you are compatible with:
If you are not compatible with Extreme, a much better option is an ethernet wifi bridge with 802.11g or later, because the encryption on 802.11b is very bad, and the speed is limited to 11 Mbps. 802.11g is 33 Mbps and can use WPA2 encryption.
Tip: Even though many G4s did not come with USB2 it is possible an upgrade card has been installed. I have 4 UBS2 ports on my computer along with the original 2 USB1 built in ports. However, with a USB2 card you may still want an external router if you put your computer to sleep. If I leave a flash drive plugged in whan putting the computer to sleep it may freeze the computer, and sometimes the card does not work after waking from sleep.