i dont think it will be a problem at all. where are you hunting? how cold will it be? 4 hours isnt that long. labs have a very warm coat and he/she shouldnt even notice it under normal circumstances. i have a GSP and they have very little coat at all and we take him in the snow all day long and he never has a problem as long as he's moving. when hu8nting, a dog works up so much heat that they rarely get cold, but then again maybe y'r hunting in -30 degrees in north dakota?