Is it possible scan dates can be this wrong

Scan showed due date was 30th dec baby come on 28th dec is it at all possible I was 4 weeks over due ? Sounds stupid I know but just checking ?
Sex march 4th period 25th march - negative test before period
Then constant sex from 2nd April- 10th April
When did it happen?

I don't get your math. If the baby was due on the 30th December and came on the 28th, that's 2 days, not four weeks. From April 2 to Dec 28 would be be 5 days short of 9 months.

If the purpose of this is to determine who is the father, this is not an accurate way to do it.