http://www.e-g-i.com/multiscreen.asp this company I have not checked out but it trades as RELL on the nasdaq, I like my own monitors and would rather use a workstation solution than buying 4 monitors just for the set up, but you can check out RELL.
Woodtronics Corporation 110 Reynolds Street S. Williamsport, PA 17702 (800) 990-3375 (570) 322-8400 Fax: (570) 327-4977 E-mail: sales@woodtronics.com
The boards I use are 2 geronimo pro cards http://www.appian.com/products/jeropro.html and a matrox PCI that controls all together 8 monitors. Three is also a solution for multiple monitors for the laptop by Appian Graphics called "the traveller" you can use multiple monitors with that as well. The board is about $300