If my electronics are outdoors in a ventillated NEMA 4 enclosure, should I conformal coat my circuit boards?
Any time you have the possibility of condensation forming on your board, you need to take precautions. It may not be that you would get a short, but corrosion instead. I'm not a huge fan of conformal coating unless I am sure I will never need to work on that board again. 
I'd rather go for a separate, water tight enclosure just for the board, so long as it's not going to cause a cost, space, or thermal issue.
That being said, coating it will probably be easier and safer.