I think the answer is yes, they'll freeze, even without the survival guide telling us this. Think about it this way. If you spend a lot of time in the freezing cold and don't move that fast you'll find that it's harder to move as your body, muscles, and bones stiffen. If they don't freeze, the weather would slow them down even more so, which makes them easy targets and less dangerous. Also, if snow is pelting them, melting then freezing, then they could literally be frozen into an ice cube. So it is my belief that the answer is yes, they can freeze or at the very least, be slowed down by the intense cold.
The only reason we dont freeze so fast is because our bodies give off heat, and since zombeis (well this version of zombies that im talking about anyways) are dead, they DONT give off heat, so at -20 and below, the zombies would probably freeze completely. Maybe this would destroy the brain (since the brain would also freeze thus damaging it) and it might kill them, but then again, no real proof that this even works.
If anything we would have to worry more about surviving ourselves, not to mention the global warming whatnot, making the more north you go less cold being more and more of a problem during a level 4 outbreak
but a level four would actually help. think about it, no workers in any plants, anywhere, the cities would be taken back by nature, and not a single plant would be operational, so all co2 emissions would stop.
Yes they would freeze. They have no core tempature. They are the same tempature as the air around them (ambient temp.). Anything below freezing for several hours and they'd grind to a halt. The perfect time to start popping heads. You'd have all winter to restock your supplies or just relocate. Come spring they'd start moving again, hopefully more worse for wear.