Remove the card, go over the gold contacts (both sides) with a pencil eraser to get them clean and remove any residue from manufaturing. Also clean inside the G4 at the slots. Then re-seat the card firmly, screwing it in helps.
Then reset the PRAM.
Not sure about the last bit, but I had identical issues with a set of Belkin 5 port USB2 cards I bought fir all my G4's. The first time I inserted the cards, 1 worked, 1 KP'd and 2 didn't see it. Things got desperate after 2 days with the 3 that didn't work (but I knew it would be fine eventually as 1 did work), and resetting PRAM was a desperate measure. Somehow the card was identified after the boot completed.
I had to reinstall the OS (which is what I should have done anyway) on the one with KP's.
See here for info on resetting PRAM and NVRAM:
http://support.apple.com/kb/ht1379
Last 2 things I can remember:
-- removing and reseating the card a few times miht have been all that was needed, just to get through any residue on the contacts on card and in the Mac, and
-- the slot the USB card goes into may be important., I tried all slots, but the success for all came with the first slot - nearest the AGP graphics slot. Now I remember reading that somewhere too, which made me try it. One of my G4 Macs had a Wireless N card already in the first slot, I had to remove it to get the USB card in, then put the wireless card in the next slot.
Let us know what you try, and what happens with each attempt. All my efforts were in OSX 10.3 and OSX 10.4.
