G4 Power Mac OS x 10.4 - how can I go wireless?

If you have a mini-tower with an open PCI slot, I would go with this:

http://eshop.macsales.com/item/Edimax/EW7722IN/

It is likely much less trouble to set up and use than finding a compatible dongle that goes in a USB port. Many dongle makers do not supply the required Mac drivers, and G4 towers are limited to USB 1.1 ports on the logic board, making the USB solution limited to approximately wireless "b" speeds. the above card will be more cost effective compared to buying a USB dongle plus then buying a PCI USB2.0 adaptor card to achieve faster wireless speeds.

The older original one did, but I think there are no drivers for the new one for PPC or OSX less than 10.6...

Compatibility

BearExtender Turbo is the perfect way to upgrade your Mac to 802.11ac. BearExtender Turbo is an 802.11ac upgrade for MacBook Pro, iMac, Mac Mini, PowerMac, Mac Pro and more.

Mac OS X 10.6 Snow Leopard
Mac OS X 10.7 Lion
Mac OS X 10.8 Mountain Lion
Mac OS X 10.9 Mavericks

Regarding OS X 10.10 Yosemite compatibility, this item is not compatible with Yosemite at this time. Existing owners please check this page for the latest beta update status.