Buy a computer that has 2 video cards in a "Crossfire" configuration with Windows 7 installed. This system should be able to run 4 monitors. As far as the monitors go, just choose the size you want in quality brands like Samsung
Just make sure your displays support DVI and HDMI, as two will use DVI connectors, and the other either DisplayPort, or HDMI.
You'll need to use Displayport for the third - or else use an active converter of some sort. A standard $15-20 converter will not work. HDMI and DVI will share the 2 available TDMI so the third has to be something else or go analog which would be ugly.