I'm not sure yet... I want to use two as a spread for sure and the other two could be setup seperate because most of the work is done on two monitors. The third and fourth display would show graphics and results of teh work done on the two main monitors.
From the THG Article AMD's New 780G Chipset In that article they used the motherboard graphics for 2 monitor and an ATI HD 3450 video card for the other two monitors for a low cost solution to run 4 monitors.
Besides the option in the article above you have quite a few options actually. Most video cards can run two monitors. And most motherboards can have two video cards - usually 1 PCI-E PowerColor HD2600Pro PCI-e $48 and 1 PCI Powercolor HD2400Pro PCI $45 for example.
The slightly more expensive Crossfire or SLI motherboards can handle two standard PCI-e video cards.
Having a display spread across two monitors is usually called Span mode (also called stretched mode).