Like AArt and others, I'm a huge fan (sorry, pun not intended) of passive ventilation just under the roof, which also helps with radiant heating from the sun beating down on your roofing materials, so I'm going to echo all the others re: expanding your existing 4" x 8' vent to 8"x 8', which is only about 5 sq ft of ventilation space, bringing your total from passive venting to about 10 1/2 ft sq (unless I missed one - need more coffee).
The other possibility on fans is going the complete opposite route. Harvest a small solar panel from something else - an Xmas light string, Halloween display, etc and connect it to a large volume air flow volume computer case fan. They are cheap, even the sealed dual ball bearing types. Solar panel on the outside, fan inside the coop, let it spin up. When it eventually goes bad after many 10s of thousands of hours use, just plug in the next one (or two or three - depending on how much air you plan to move.
Since you are dealing with extremely low voltages, its safe even for those not comfortable around electric circuits.
