I got such a card, except I think it's ATI, and there exists a way to "hack" the ADC power to connect to the G4's ADC slot which involves taking a chunk of ISA card of approximately the same size and soldering it to the cards misplaced connector and offsetting it with wires so that it fits into the slot properly.
I bookmarked it somewhere, and I'll post it up here at some point if I find it.
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EDIT: Also, pins 3 and 11 have to be taped off or disconnected somehow on some cards, otherwise they interfere with the G4's AGP slot, causing a no video condition.