4 weeks pregnant but ultrasound shows no sac?

It is possible that you had a miscarriage and there is a little bit of the pregnancy hormone left in your body. I am guessing you were actively trying to get pregnant and looking out for it, otherwise you might not even have known you were pregnant. Sometimes the body just naturally rejects something that is not forming correctly. I had a miscarriage at 6.5 weeks but got pregnant 3 months later and now I am 27 weeks pregnant and everything is going really well. Best of wishes to you, I am sure that if it doesn't work out this time it will work out the next time.