Sex without protection on the 4th day of my period, bf was inside less than a minute and didn't finish til way later, 27-30d cycles. Pregnancy chance?

Yes, but low. Pregnancy is know to have occurred from sex on any and all days of the cycle. If you are within 72 hours of sex, you may take Plan B to prevent pregnancy. If it is too late for that and you miss your period by 4-5 days, do a pregnancy test. If the result is negative repeat the test 4-5 days later unless you get a period by then. Use first morning urine and follow the instructions for the test carefully. If you do not wish to be pregnant, use contraception all the time, every time. You may consider implanted contraceptive, or IUD. Practice safe sex.