For a 4-way SLI setup, I have seen mixed result when the 4th card is selected as Dedicated Physx Card in the Control Panel.
This person reported that doing so would turn the setup from 4-way to 2-way, with the 3rd and 4th cards taken out of the SLI (possibly resulting the 3rd card idling and the 4th card as Dedicated Physx card).
This person showed that doing so would still keep the 4-way SLI intact, just that the 4th card doing rendering and Physx. See diagram below for example.
Both persons are using GTX480s as late as Dec 2010 and Jan 2011.
Here is an 'outdated' article (Nov 2009) but should still be educational.
Personally, I think the sure and easy way to change from 4-way SLI to 3-way SLI is:-
a. Click 'Disable SLI' in Control Panel
b. Power off
c. Remove the 4-way SLI bridge
d. Insert a 3-way SLI bridge
e. Boot up
f. Go into Control Panel and Enable 3-way SLI
(hopefully a re-installation of driver is not needed)
There are a few members here running 4-way SLI setup. They surely can confirm this easily with their rig.