Thanks for your question.
you haven't mentioned when was your last scan done if it was done recently & there is a four weeks gap between the duration of gestation & ultrasound
measurements then you baby is definitely small for dates. This means baby is not growing properly & there is some defect in the nutritional supply to the baby. You need extra surveillance & care led by a consultant obstetrician. Doctor will assess your baby on frequent basis both clinically & by doing tests to assure a healthy outcome & you may need an early delivery to save your baby. Your baby may also need nursery care for some times.
Best of luck.