Hi - At 4 weeks preg you def cant see bubs... you will see more at 12 week scan. I had a 'dating scan' at 8 weeks and all i could see was a little blur, I would not have seen a think except the sonographer pointed it out to me. At 8 weeks I was told they may or may not pick up the heartbeat - they did but this is quite early.
I would say not to worry, see what the blood tests say later in the week. The U/S cant tell you muchmore and they have just gone through what the options could possibly be, no indication that you have m/c or ectopic.
Best of luck!