They can run four monitors in extended desktop mode, which means that it will be just like having a larger desktop.
However you wont be able to game across the screens, so you couldn't do something like this for instance.
If you switched to a Radeon card you could (if one of your monitors is DisplayPort capable, if not you will need an adpter) game across three screens, but as I said before, you would need at least two high end cards to get good performance at that resolution.
Also, for either Nvidia Surround or AMD Eyefinity all the monitors need to be the same resolution. So if your using mis-matched monitors you wont be able to game across them, but extended desktop is still possible.