not sure what you mean. the 9600 pro first shipped with G5s and there is no ATI support for it in OS9.
so why spend 120 for a card when you can get the same card by buying a G5 for less?
G5 cards are AGP PRO, not 4x AGP, so unless you plan on taking a hacksaw to the G5 card it won't fit in the G4's AGP 4x slot.
Some people have been able to do such things with cards that don't really need the extra power supplied by the pins you have to cut off the connector.
Tho the R9600 XT came with a standard non "Pro" connector with 128MB VRAM and can be used in some G4's when taping pins 3 and 11, it doesn't support Dual-Link DVI. The R9600 Mac/PC Edition has 3 external TMDS's and can drive one Dual-Link DVI display as well as a standard DVI Single-Link display and has a 256MB VRAM.
There is a Native Device Driver( 'NDRV' ) for OS 9 support, you just don't get any 2D/3D GPU acceleration, but you can drive displays at insane resolutions, and that's the whole point of this thread, more desktop spaces when using static windows.
There was a R9650 that had dual dvi and can drive one single link DVI and one Dual-Link DVI with 265MB VRAM, but good luck finding one for less than $120, tho they do pop up from time to time at good prices.