You have a couple of options depending on what you want to do. If you are only looking to game on one monitor, but have 4 connected all at the same time, then you can get by with a single ATI card that has enough displayports on the card. I know there were some high end ATI cards that have 6 display ports on one card allowing you to connect up to 6 monitors at one time. Something like this would work for you: http://www.newegg.com/Product/Produc...-888-_-Product
If you want to span your games accross all 4 monitors and use them like they are all one large monitor, you will probably need a multi-GPU setup. The card above could techincally handle the task of spanning your game for you, however, it might not have enough horsepower to actually play newer games at high settings accross all those monitors. You are probably looking at a minimum of 2 graphics cards for that job. And really, you have a lot more options going this route. You can get two nvidia GTX 500 series cards and do nvidia surround. Or you can get 2 ATI cards in crossfire and use their eyefinity technology to achieve the same result. You will want some pretty powerful cards for gaming accross 4 monitors because your resolutions would be very high. I would consider something like a pair of ATI 6970s because they offer more video RAM then the GTX570, and at those resolutions, that will really help you out.