If You Don't Have Four Ports
If your system lacks enough expansion slots to hold two graphics boards (for instance, because it's a laptop) or you simply would rather not open your PC's case, you can still enjoy the benefits of a quad-head display.
Display Link makes adapters that allow you to connect a monitor to a computer through a USB port. It works by emulating a graphics adapter in the computer's CPU and then sending out a compressed data stream to an attached adapter that decodes the data and transforms it back into an image signal. Some companies have built the decoding circuitry right into a monitor, such as the 22-inch LG Electronics L226WU-PX, so all you have to do is plug your secondary monitor directly into a USB port.
You can also get adapters that use DisplayLink technology to turn your USB port into a DVI port for use with any DVI-equipped display. Following the instructions, install the DisplayLink software before connecting the adapters. After the drivers are installed, you can plug the monitor into the adapter and insert the adapter into a USB port, and you'll then see the Windows desktop on the screen. Follow the same steps to extend the Windows desktop across the extra screens and to set the resolution to match each panel's native resolution. No expansion cards, no muss, no fuss.
Whether you use adapters with a laptop or work with installed graphics boards in your PC, you can quickly and easily set up a quad-monitor system for your computer. Assuming that you already have one 19-inch monitor and a dual-head graphics card installed, you can build up to a 38-inch diagonal display for under $1000 total, and get the big picture for all your Windows work (and play).