Oh, there are certainly some. They're normally just quite expensive.
Don't think I'm trying to deter you!
Just letting you know that quad xfire/sli set ups are very costly.
You'd more than likely also have to get a full atx case.
A 1200W PSU should do the trick for a quad setup though!
(another note, most boards with 4 PCIE slots that aren't crazy expensive drop speed when you have cards in 2 or more slots).
Annnddd by the time you'd get to the point where you'd upgrade to quad cards, new tech is going to be out that you're probably going to want instead!