Yes, Reaper does midi control channels.
There is also a section in the pdf manual on how to set up control surfaces.
I'd read it and then talk to the guys over on the Reaper board that use them....
($700 seems a bit steep for 4 faders tho when you could get something like the Behringer BCF2000 B-Control Fader for less than $200... again, talk to the guys over on the Reaper board to see what they recommend.)