any 40x40x10mm fan will work, but using 12v on 4s the fan might fail early, but if you get a 40x40x10mm 5v fan, wired to a servo plug and plugged into a spare channel on your rec. will run the fan at 6v in stead of 5v...and it will draw more air to the heatsink... here is a link to fan
https://www.eagepc.com/product_detai...id=276&id=4478
the fan is a 40x40x7mm and it has 3 wires....just cut off the yellow wire..& use the red & black to power fan