If multiple early ultrasounds & subsequent ultrasounds yield the same edd is it possible for them to be 4 weeks off (4 weeks further ahead)?

Well: Lets say it is low probability. Early on, in the first trimester, a measurement called the crown-rump length is taken. This is a head to baby's rump measurement. This is so accurate that if the due date from the last period is more than 5 days off the ultrasound due date is used. Behaving multiple confirming scans is even more reassuring that the due date is accurate. Good luck.