I had 4 regular periods since i had protected sex last time..this month its a week late...are those 4 periods 1st trimester bleeding?what are the chances of pregnancy??

If the "four periods" occur at 24-32 day intervals, and no subsequent sexual exposure has occurred since referenced in your question, the chances of pregnancy are minimal, and the "periods" are not related to "first TM bleeding".

Your chance of pregnancy after protected sex is low. Your chance of having 4 months of normal menstrual blood flow during pregnancy is very low. Nothing is “never”, but pregnancy with these criteria is highly unlikely. If still worried, check a pregnancy test.

It is very very highly unlikely that you are pregnant. You may do a home pregnancy test. Your doctor can do an ultrasound examination that will clearly show a fetus at 4 months after sex, if you were pregnant. If you do not wish to be pregnant, use contraception all the time, every time. Consider implanted contraceptive, or IUD. Practice safe sex. Get HPV vaccine,