I have always been told that ultrasounds can be off by a week or so, especially in the beginning when the baby is so small.
I think that 15 wks and 4 days vs 16 wks is a negligible difference. I wouldn't worry too much.
From what I have heared, ultrasounds aren't always correct. I should have been 8 weeks 6 days during my first ultrasound based on my last period, but based on the size it measured 3 days earlier at 8 weeks 3 days. Same with weight, they can be off with that also. I don't really know much more than hear-say, but if the doctor isn't concerned and you trust their opinion then I would just go with that.