4 screens
The matrox box does do three screens. If your graphics card can support two monitors (ie two screen plug in points on the card rather than one), you can use the second interface to run the forth screen. If you really want to go to town you can plug a second matrox box into the second interface and have a total of 6 screens!! ;-) but 4 is enough I think.
The FSX weather addon mentioned above, works very well and should be just the ticket.
BP.