Hi! Colorado runner here 🙂 First, I think it's all relative! Is 40 pretty cold for your area? 40, for me right now (CO winter), isn't too bad if the sun is out! My general rules-of-thumb with running attire and weather:
50-60 degrees: Shorts & long-sleeve (...over 60, I'll do shorts and a tank). If it's windy I'll wear a headband over my ears.
40-50 degrees: probably crops or pants, long-sleeve, maybe a vest if it's windy or the sun isn't all the way up yet, and a headband. Maybe gloves, but usually my hands get warm too quickly.
30-40: pants, long-sleeve, vest or pull-over, gloves, headband 🙂
I get hot quickly - even if it's freezing outside (it was 0 here the other day). But what I do is put on a form fitting jacket (so it doesn't flop around and distract me) and maybe hat / gloves and do a quick lap (1/2 mi to a mi for me) around my block and then dump all of my gear off (cause by this time I'm overheating) back at my house and go for my run! Gets me acclimated and warmed up without having to worry about carrying everything with me if I get too hot.