The earlier scans are the most accurate. Some say ur first scan at about 7wks is most accurate, others say the 12 wk one is, but definitely by the 19wk scan baby is displaying his or her own personal traits ie long or short legs, big head small head etc etc, so that scan and any scans after are not accurate for dating (more just for checking baby is growing steadily by comparing to last scan's measurements and for diagnostic reasons). This was told to me by a sonographer.
It also depends on when you had your scan and when your baby had a growth spurt...........they might measure 5 days behind but if they were measured 2 days later they might measure spot on.

Personally i would use your 2nd scan dates because having your 1st scan at 4 to 5 wks, i would say this is too early to get an accurate measurement.........had it been at 6 to 7 wks then i would have said use those scan results (im assuming your 2nd scan was the 12 wk scan).

Dont stress about baby measuring a week behind for the last 4 scans (mine would measure well over a week behind from about 30wks on, they are small kids but healthy), baby is keeping up being a week behind which shows baby is growing steadily