Can you make sure that the card is enabled in Windows?
Right-click on the desktop, edit Screen Resolution, and verify that you have "Extend Desktop to this Monitor" enabled (instructions are from memory since I'm on XP atm).
Windows and Catalyst Software will detect all 4 cards, GUIMiner refuses to load when I have 4 cards on. 3 Cards it'll load up just fine.
Do you have all 4 connected to some sort of display? Or at elast some dummy connection for the 4th card?