Ay, ganyan talaga sometimes! A 4-week gap is actually pretty common, lalo na kung late ka mag-ultrasound or kung irregular ang cycles mo, kaya ang dating ng conception ay mahirap i-pinpoint exactly ng doctors. Best advice is to ask your OB directly kung bakit ang difference, kasi they'll have your full medical history at can explain kung saan nanggaling yung discrepancy, at which date ang reliable para sa kanila.