Pregnancy can be counted from the first day of last menstrual periods or from the day of conception. Exact conception is difficult to calculate because it depends upon ovulation which can occur at mid of the cycle but early and late ovulation is also possible. Sperms can also stay alive for 5-7 days in female body, so, when ovulation took place and when fertilization has occurred is not possible to know. It is only assumed that it took place at mid of the cycle in a 28 days cycle. That is why pregnancy is usually calculated from the first day of periods not from the conception date. If your doctor is counting pregnancy from first day of last menstrual period then you conceived 4 weeks before and if he is counting pregnancy from then day of conception then it took place 4 1/3 weeks before.