If a baby is measuring 4 days bigger on an ultrasound but the doctor didn't change the due date what does that mean?

Normal: It depends when you had the ultrasound because with all ultrasounds there is a range, like a 3rd trimester us can be off a couple of weeks either way.

No need: The measurement error of 1st trimester us is within +/- 4-5 days, so being 4 days different from the calculated date doesn't really mean anything. The calculated date or the us date could be the due date.