That warm? I would stick with a 2P. Any advantage you might gain in the way of warmth would be outweighed by the loss of space, as noted immediately above. When the GF and I did supported tours, we always took the 4P. Didn't regret it one bit, including on Cycle Oregon when we woke up to ice on the tyent on two spate morning.
Ifr you re really cold, wear the jacket. Don't drape it over the bag.