Cost isn't the issue, size is. The only chiller that I think would remotely fit would be the MCW-CHILL, but the only problem there is would it be able to handle the amount of heat being generated by all 4 CPUs??
--Edit--
After thinking about this abit more, what about connecting an MCW-CHILL to the res to cool the water in there?? I have attached an image so that you guys can see what i'm thinking. Everything in Black is the main WC loop for the two computers, and the stuff in Red is the WC loops for the chiller..