If they told you on January 1 that you were at 4 weeks 5 days GA, and if your last period began on December 2, the count would not have been based on your last period but on your ultrasound measurements, because counting from your last period would have been 5 weeks 2 days or thereabouts.
Anyway, nothing in your information suggests that the baby was four weeks more developed than they were telling you. It sounds like they accurately represented what they saw in the ultrasound.
If all they saw was a fetal pole, 4-weeks plus is about right. An 8th-week embryo has arms, legs, and head, looks like a little teddy bear and is often swimming around. If you want, look up photos of embryos in the 4th week and the 8th week. They look worlds different.
Also, if guy 1 had been the father, all your subsequent ultrasounds would have shown this by the baby being that much further developed, and the baby would not have weighed only 5 lbs. 2 ounces when born.
Let me correct some things
I’m not sure if the due date was dated from the ultrasound or my last period and I also had another ultrasound around January 11-12 and it was measuring 6 weeks but nothing was visible yet in the ultrasound. If that helps at all.