This is the fan that I use. It's plugged into the 4520's rear USB port and turns on when the receiver powers on. I just lay the fan over the warmest spot on the receiver's top and the receiver stays nice and lukewarm. You can direct the exhaust air in the direction that is most appropriate for your setup. The fan is pretty quiet but it can be heard once you get within a couple of feet of it. I never had a problem with the receiver overheating, in fact this Denon runs much, much cooler than my previous Onkyo 705, but I figured this fan would give me some cheap piece of mind.
I think that larger, slower, quieter fans are always better, so I have one Silverstone AP181 sitting right on top of my receiver; it is powered with a USB to 3-pin fan adapter. AP181 is not a blower design, so it does require some clearance above it if used as an exhaust fan. AP181 is 18 cm in diameter and is extremely quiet at the lower speed setting of 700 RPM (18 dBA according to Silverstone).