I am confused about how the weeks of pregnancy are calculated. If conception occurs in week 3 of pregnancy, how could week 4 be 2 weeks post-conception? How are the weeks of pregnancy typically determined?

Menstrual Weeks. Usually, the actual date of conception is unknown, therefore pregnancy is dated based on the first day of the last period. The due date is 40 weeks from this date. Sometimes conception date is known (ie ivf pregnancy) in which case the estimated due date is 38 weeks later; for the purposes of calculation ovulation is considered as day 14 of a theoretical cycle. Early sono will confirm dating.
Misconception. Conception can not occur during pregnancy, not in the 3rd or any other week; it rather starts it. The other idea is correct, conception occurs in mid-cycle, and then the 4th week of that period is 2 weeks post-conception.