Courtz I feel for you. I went for a scan through the EPU at what I thought was about 4-5 weeks as I was unsure about dates. All I saw was an empty sac. No yolk sac, no foetal pole, no HB. I was told that it could be too early, but the woman was horrible and said it most likely would result in a miscarriage. I cried for hours. (It didn't, btw!)
When I got home I researched everything surrounding ultrasounds around this time of gestation. From what I remember off the top of my head -
Basically, 4-5 weeks is so so early, and to even see a yolk sac is impressive! Typically a foetal pole and heart beat aren't usually visible on an ultrasound until about 6 weeks. Sometimes a HB isn't found until closer to 7 weeks.
In two weeks, when you go back, you should be able to see the foetal pole and HB on the scan if you're already closer to 5 weeks at the moment. It could also be that your dates are a little off and you're actually closer to 4 weeks so you're a couple of days out of being able to see baby yet!
I spent the whole week before my rescan during that time panicking and worrying and crying, so I know it's scary, but try to keep calm. It's so early.
It really helped me to google some statistics and watch them go down day by day, and also look at scans and what is expected during what dates. It helped me rationalise a lot and understand why they were so mean about viability and miscarriages!
Fingers crossed for you in the next two weeks! x