OK, we got that cleared up! I was thinking you were taking the cooling fan off of your computer, and hooking it to your radiator.
Anywho, 4" thick should be no problem. Think Black Magic, or honestly junkyard. I solved my cooling problems with a 16" (diameter) fan off of a 4.0L Jeep.
Be creative, take a walk through the junkyard instead of the Summit catalog. You'll come up with it. I've found that surface area that the fan covers is more important than the CFM it flows.
