I have your exact model of G4, it's a 1.0ghz dual MDD, and I use a D-link DWL-G122. They're about $39.99, and while supposed to not work with macs, it uses a Ralink chipset, and mac drivers ARE available for this. It's only 802.11g, but it's still plenty fast. It's a USB adapter, and if you have a laptop, that will work on it, too. You will need to be running 10.3.x or 10.4.x for it to work. And it MUST BE HARDWARE VERSION B1! THIS IS VERY IMPORTANT! If it is not, then it does not have the Ralink chipset, which means that it won't work. I got mine from Radioshack, but Target has it as well, though Target's price is $49.99 Either way it's cheaper than the original airport card, and it's faster too. I get 54mbps connection rates, so it's quite impressive. I do have a USB 2.0 card in it, but of course, this isn't necassary, it will work on USB 1.1, albeit slower. I would recommend buying the D-link adapter, it should work with it. The Hardware Version will be stated on the box, so I recommend buying it in a store if possible, as then you can be sure if it is the correct one. I imagine someone will come out with a 802.11n adapter in a little while, but even then it may not be compatible with G4s. I would just buy the D-Link, but that's just my 2 cents worth. If you search for "DWL-G122 Mac" on google, you'll find a guide that tells you exactly how to do it
God Bless!
John