Intake/exhaust sizes?

Hey folks,

I have a four inch exhaust vent with an inline fan exhausting into my room out of the grow closet. I cut four inch vent holes in the back of the closet into the attic, which has clear airflow from the outside. I have heard that passive intake diameter should equal twice the exhaust vent diameter. However, I live in an area where it will be hovering around 0 degrees fahrenheit for much of the next 3-4 months, and I'm worried about it getting too cold as it's already around 74 during light on and 67 light off (and I'm still on 20/4). So can I just cover up and re-insulate one of the intake vents if it gets too cold? Wouldn't this just make a faster stream of cold air come in from the one remaining intake vent? Maybe it would be better to just time the exhaust fan to only work half hour every hour somehow? Any input much appreciated.

you can always buy a fan controller that has a thermostat (http://www.plantlightinghydroponics.com/cooling-thermostat-for-ventilation-fans-p-1488.html). There a bit pricy, but due the the extreme cold you will be experiencing, it might be worth the investment. you can even get backdraft damper (add a screen or filter to it) for when the fan is not on to limit the amount of cold air falling into the grow room (http://www.plantlightinghydroponics.com/canfan-inch-backdraft-damper-p-2439.html). the parts will save you time, and keep the guess work in and when to run the fan.