I agree with Cinna696 in that we could use some more details.  However, I completely disagree with the :evil: way he asked.
I don't think there is a video card that can output to 4 monitors, so you'll have to go with two dual display cards.  Gaming performance likely isn't an issue, so you could go with an entry level AGP or PCI Express (depending on the motherboard you pick) and a PCI video card.  You could also go with two PCI Express cards.
A kind of strange way to do it would be to use the ASRock Dual939 motherboard, and put in a PCI Express and a AGP video card. 
So, details we need to know:  What motherboard will you be using?  Are these 4 monitors going to be VGA or DVI?  Will there be any gaming on this system? 
Edit: I stand corrected, I guess 
this will drive 4 monitors.  However, it is expensive and out of stock 
