if you have a regular 28day cycle, the chances of gettign pregnant day 4 of your period are slim - however, one week after the start of your period. 1 wk after your period ended (if it was a 5 day period, about Cycle Day 12) would be optimum time for trying to concieve, as in a regular 28 day cycle, Ovulation is usually around Cycle Day 14. So if you are late for your period this cycle, take a pregnancy test
the 4th day of your cycle you are highly unlikely to get pregnant, however about a week after your period ends most women are very fertile and have a good chance of getting pregnant during that time.
Anytime you have unprotected sex you run the risk of pregnancy. If you do not wish to be pregnant than you need to get onto some type of birth control and not engage in sexual activities unprotected.