4 weeks pregnant would be too early to see anything on the ultrasound, 6 weeks you should see something. With healthy hCG numbers doubling well like this, let's hope it's all there next time and you were just a little bit early. If in a week or two weeks there is still nothing in the sac but your numbers are still rising, only then would I begin worrying about ectopic pregnancy. Cross that bridge when you come to it, though. You may just have ovulated a little later in December than the average, and the embryo will be visible next time.
This is my first pregnancy. My last period was December 10th. I'm in a similar situation; however they are taking my hcg levels today and then again on Wednesday. I pray that we will both be just fine. Keep us posted.