Hi welcome to H.C.M.Forum. you are a human not a machine, there is every chance of variation in every event. ovulation usually can occur from 10th day to 20th day, some times even before or after that also. the sperm should be healthy and reach the ovum on the day of ovulation. it might be pregnancy
but may not of 4 weeks duration. on 4th it is positive means it might be of 9 to 10 days duration.there is risk of tubal pregnancy also, be cautious if you get very severe abdominal pain
. let us hope that every thing will be fine. thank you .