What is the screenshot? I spoke with a friend who had similar problems and he suggested that a different monitor might work on top. So, I tried all different configurations - no luck. However, I did notice that it works fine with two monitors on top and two on the bottom. Really odd!
The driver is probably more accommadative to that setup. You probably have to live with what Windows and the driver will let you.
The screen shot is of 3 monitors on the bottom and 1 on top of the middle one. As you can see, the top left and right are blank as there are no monitors there. You can see the setup in the botton right hand screen as I have left the Display Properties dialog box up. Windows and my driver for the Matrox G450 can handle this setup and I so think your driver is lacking. Hope this helps.
Many thanks for all the advice. I will let you know if I ever can figure this out (other than just buying another 19 inch monitor). Have a great holiday!