Thanks for query,
If she has a period though light & you have a sex after 4 days of it then it falls under safe period & there are almost nil chances of being pregnant .
Ovulation occurs mid cycle & ovum can live up to 3=4 days while sperm intrduced in female can live for 2=3 days.
In a 28 days cycle ovulation occurs on 14 th day so first 10 days & last 10 days are considered as safe period during which you have sex no pregnancy
Your sex falls under this period so nothing to worry about pregnancy.