Quad monitors is unfortunately always a tricky proposition and will almost certainly require a bit of fettling with the xorg.conf file. I had it all working fine under 11.04 but after upgrading to 11.10 things went down hill. There is a description of the problems I had here : Upgrade To 11.10 - Issue With Quad Monitors
I am using two different ATI cards and the xorg.conf I had used to work fine. However, under 11.10 things seem a lot trickier. I have achieved varying levels of success and I did manage to get four monitors working temporarily but the system constantly froze and X would restart every 5 minutes or so.
I have since had a chat with the chaps over on #radeon on irc.freenode.net and they have assured me that my config is correct (which would seem to be correct as it worked under previous versions). They suspect a bug in Gnome or X is causing the problems.
Anyway, long story short. If I was in your position I would not expect even two identical cards to work by simply plugging them in and it all magically works. Expect to have to put a bit of work in getting your X configuration perfect manually. It is quite possible that there is a configuration out there that will work out of the box but until you have seen someone do it, plug the cards in and boot up with all four monitors on, be prepared for a bit of a hard time...