What are the chances of Blighted ovum?

4 weeks is still early to see things on U/S. Sometimes a tiny gestational sac will be seen during the 4th week, but a yolk sac isn't seen in a sac until the gestational age is 5 weeks. While there is always a possibility that an empty sac may develop, a 4 week scan would be too early to tell. They will be able to tell you for sure on the 8 week scan. By then, a heart rate should be able to be found (if not by the little doppler machine in the office, then by Ultrasound).