The G4 Powermac only ever came with AGP graphics, I beleive the best card you can get is a dual-head 8500... perhaps there is a 9800 available as well.
Unfortunately you can't get anything more powerful than that in the G4. A G5 can handle multiple PCIe cards, and the Mac Pro can take up to 4 GeForce 7300 cards for 8 screen outputs.
I'm not entirely sure, but I think if you find out what PCI graphics card came with the early G4 PowerMacs and buy up a few of them you should be able to plug in some more screens. Trouble is, none of them are dual-head. It's only the later Radeon cards like the 8500, 9800 that had dual head capability.
Regarding the above I'm 98% sure. Go to lowendmac for some more info.