I just wore this as a shirt in fall and wear it underneath my clothes in winter and it's pretty warm. I think I might get the 4.0 because our wind chill gets down to -40 but with this and a big jacket and an average winter temp around 0, this does the trick. It is super staticy though after it comes out of the dryer. All of the dog/cat hair and dryer sheets stick right to it so it feels impossible to get really clean.