There is a possibility that they didn't remove everything in the D&C. Or it could still be early. I had a D&C a year ago for a m/c and I can't remember if I was still showing + at 4 weeks. It was about 8 weeks before I got my period back, so I may have been.
Was it a bad procedure? Do you hope you are pregnant? Why did you have to have a D&C?