well 4 screens mean two catalyst to make it simple .
Yes you can use 2 dual head 2go from matrox , but you will loose all the control of the projectors .
It all depend of the projectors you want to use and how you want to use it .
Btw , If I ware you I'll buy the following gear :
1 Macpro 2,66/2 giga ram http://www.apple.com/macpro/
1 Ati radeon 1900 xt http://ati.de/products/RadeonX1900/index.html
2 matrox dual head 2go
1 MTRON SATA 32GB SSD http://www.king-cart.com/cgi-bin/car...ct_match=exact
1 Artistic license's Cata-Lynx (Cata-Lynx) http://www.artisticlicence.com/item371.htm ONLY IF DMX NEEDED
1 Black Magic HD http://www.blackmagic-design.com/products/hd/
1 mac book for remote control
0 local monitor
0 vga splitters
this is just my opinion , listen to others b4 buy anything .