ATI's IGPs support a feature called surround view. When enabled in the BIOS, if an ATI video card is installed, the onboard video stays active and can be used for multi-display output. I've seen it used for three digital displays (two from a discrete video card and one from the onboard)... it may work with four displays though I've not seen that tried.
I had an older RS482 motherboard that supported the feature and I have an Asus M3A78-EM (780G) board that also has the option in the BIOS. Just turn it on, pop in a Radeon of some sort and then configure the multiple displays with Catalyst Control Center.