Yup, if that 4x12 is in inches, that is *radically* insufficient ventilation.
If you haven't already bought anything, though, can I suggest DON'T BUY VENTS. Almost anything you can purchase will be simply too small to be of much use, in which case you put in a bajillion of them, which is more work and expense than just making the vents yourself.
All you need to do is cut holes in the coop wall, at least the downwind one(s) and preferably others too, high on the walls (hopefully up where they're protected a bit by roof overhang). I would suggest something like a 1-2' high opening that runs the whole length of the wall, or equivalent area in whatever other arrangement you use. Then make flaps out of plywood to cover the openings in an adjustable way, so you can change how much of the vent is how-open according to the weather and coop conditions. Cover the openings with hardwarecloth to keep predators out, and there you go.
Good luck, have fun,
Pat