Ob-Gyn questions? Ask a gynecologist online.
I'm Dr Manos obgyn. I did thousands of my own transvaginal scans and evaluated many quantitative pregnancy values. It's a good sign that the hug level at least doubled in 48 hrs. A four week sac is a little early to see evidence of any fetal echoes. There's a good chance for the next scan showing a fetal heart flicker and yolk sac both being reassuring that all is well. Some of the spotting could have been implantation bleeding that occurs when the pregnancy embryo is settling into the lining of the uterus and as a sac develops there is some blood discharged from the peripheral margins of the sac on the lining.
6 weeks is measured back to LMP The sac of 4 wks is from conception or 6 wks from LMP. All that matters right now is the next ultrasound.
You have a sac in the uterus and you will know more of what's going on with the next ultrasound. Don't try to figure it out with what you know about LMP and conception date. Be patient for now.