Really 4.5 if I could do it. This layer is beyond warm, especially for how light it is. I was wearing it over a t-shirt in 40 degrees yesterday and was just slightly on the cold side. About the same as I feel in my pea coat! Great stretch and super soft. Collar stiffness is just right for me. Fit is thin but not tight. I'm a lean (not skinny), athletic 5'6", 140lbs, with a 37" chest and the small fit me perfectly. No complaints about sleeve length (and I usually wear a "short" in formal jackets). There is no stretch when my arms are over my head and the snug wrists ensure that the arms bunch nicely when my arms are down. This is coming with my to Denali this summer.
Small complaints: The zipper is longer than I'd like. I find that it makes the shirt look a little funny half-zipped. The fabric really shows the texture of what's under it. Don't even think about keeping it around as an all-purpose layer for the office as you will look ridiculous if your shirt has buttons or pockets. Even with a t-shirt under, you're still going to get little peaks over your nipples (peaks that don't show up when just wearing my t-shirt).