One of my friend has intercourse only on 4th day of her cycle an no intercources on that cycle. she was conceived and now boy. who is this possible.?

Anything is possible: It depends on the day you start counting. Usually, day 1=period start, day 14=ovulation. (how odd to only have sex on one set day a month!) if she has a week-long period, then starts counting 4 days, we are up to day 11. Sperm may be stored in cervical crypts and live 48h. If she ovulates a little early (day12 or 13), then she conceivably get pregnant!