You may simply be out of luck with that arrangement. I'm not aware of any multi-monitor utility that allows arranging the monitors in other than a uniform row/column arrangement -- i.e. either all in a row; all in a column; or in a rectangular array of m x n rows and columns.
In your case, something like 1-2-3-4 or 1 - 2
3 - 4
Even the very-powerful ZenView manager only allows those configurations [
http://www.digitaltigers.com/zenview.asp ]