Not sure. With my son they saw just a sac when I was 5 weeks. Everything is so tiny at this point that it's hard to get exact measurements. At 4 weeks though I think they can tell if you're pregnant by the lining in your uterus.
I think you need to be 5 1/2 weeks or thereabouts to really be able to confirm your pregnancy is viable. Saying that, if they don't see anything on your 4 week scan, wouldn't they bring you in a week later to make sure it's growing? That's been my experience at least.
Congrats on your pregnancy! And to all the new ladies too! This new reality is still sinking in for me.