Quote from GoldStandard:
There are basicly two ways you can go: One way is to buy a newer AMD eyefinity card and buy 4 new monitors that all support the Displayport standard. Then you daisy-chain the monitors together and connect them to the displayport port on your video card. The disadvantage here is that displayport monitors are considerably more expensive.
Or you can go with a 2-card solution, assuming you have a spare pci slot in your computer. Buy 2 video cards that can each support 2 monitors and connect 2 monitors to each card. Make sure the 2nd card you buy matches the type of pci slot you have free. Unless you already have 4 displayport monitors , this will probably be the more affordable solution.