Negative.
The G3 Beige through the QuickSilver are technically 'based' on ATX, but in practice have very little in common with standard ATX other than the basic shape and orientation of the motherboard.
You'll be able to modify a standard ATX motherboard plate to accomodate a G4 board, but you will have to redrill your motherboard mounts. If you're lucky, one or two of the mounts in your case might line up with the G4 board. But you're gonna end up drilling.
Such was the case when I put my old Digital Audio board into a Kingwin aluminium case a few years back.
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