It's simple to modify these cards to work in a G4...put a small piece of tape to cover pins #3 and #11 on the bottom of the card and the card is forced to function in 4x mode. If you're planning on upgrading though, the 9800 tends to be a much more powerful card....I was running a 9600 from OWC, then did some work with a 9800, and for newer games, wow...huge improvement!
Pin 3 and 11 are found by turning the card upside down (so the heatsink is facing down) and counting from the back of the card towards the front (back of the card is side the monitor connectors are not).
wangahrah