You need to go to the Nvidia Drivers Download web site, and download and install the 4000 driver for OS X 10.6.8, which will be the 256.02.25f01 driver (the last time I checked).
Also make sure you have the Aux Power connector connected to the 4000 card. Lacking that will also cause the card to not be recognized.
Dwaine Maggart
Blackmagic Design DaVinci Support