4 weeks is very early. We had to do vaginal ultrasounds until I was about 12 weeks because of where it implanted.. The ultrasound machine couldn't see the baby in abdominal ultrasounds til way late it seems. It's normal for it to only be a sack at this point. As long as it continues growing and developing you should be fine. Congrats! :)
At 4 weeks its too small. They didnt see my baby til 6 weeks... I am now 33 weeks with a healthy baby boy.
Usually they don't do ultrasounds until 9 weeks because you won't be able to see the baby and they don't want to freak moms out.
Way normal. 5wks baby is a sesame seed so at 4wks theres nothing visible yet, and not even a heartbeat.
It's very possible. I wouldn't worry too much