At 4 weeks it should be 300-400 and double every 48 hrs right to start with. At some point it takes more like 3 or 4 days to double, but I'm not sure exactly when that starts. More importantly than the exact numbers, though, is how fast they are going up.
2 yolks, 2 sacs is twins isn't it?
I had a scan at 6 weeks and had the same - only one sac. It just meant that it was two early. Went back at 7 weeks to be sure and there was a blob - aka my baby!
I wouldn't worry :)
I went in at 5 weeks and they did not hear a heart beat I was told that it might be molar pregnancy and that I would have to get a dnc I was asked to go back in a week. To make this short I could not go back I felt it was to early and needed to give my baby a chance to grow. I went back at 8 weeks and found out it was twins. Best of luck to you hope all goes good for you too.