I suspect the date they gave you was actual age/gestation of the baby (so counting from conception) rather than a traditional pregnancy date (counting from your last period). So if you add 2 weeks (to take it to your last period), the date makes sense. I very much doubt that they would have seen anything at all if you were only 4 wks.
Definitely worth getting a blood test to check your HCG level and potentially another scan. I think knowing whether or not the pregnancy is viable is a good first step in making your final decision,