4 weeks pregnant - nothing on scan

hi guys. my friend had a positive pregnancy test and her docs scheduled her in for a scan at what would be 4 weeks (don't know why so early), which was today. They couldn't see anything on the ultrasound and now she's worried she's lost the baby. I just wondered if I could have some reassurance to give her hope that it's because it's too soon and not because of the worst. thanks in advance