What are the chances that my girlfrien gets pregnant while shes 4 days into her menstrual cycle?

Do you want a baby?: Assuming your girlfriend is also a teenager, I am guessing that you are not trying to become parents. There is a chance of pregnancy with sex at anytime because it is impossible to 100% accurately predict ovulation. So, even though day 4 is less likely, then day 13 - there is a chance and that is a good reason to use birth control. STD protection is also important! Use a condom. Take care.