I am not at all medically trained, so see your doctor if symptoms persist etc,... but it sounds like cyanosis- blue lips,hands, feet etc, which can indicate a lack of oxygen in the bloodstream. This is serious if combined with/caused by other health problems i.e very bad cough indicating that breathing problems are dangerous-causing oxygen shortage. In the case of your 4 year old, it's probably normal and fine as when in cold conditions the body moves blood from the extremeties to the bodies core to preserve heat. This results in less blood in areas like the face, arms and legs, which produces the blue colour. The opposite of this is of course red flushed cheeks, which occur when the body is trying to loose heat.