Will having sex one day before my expected period get me pregnant? I'm already 4 days late now. Help! :(?

There is a possibility but not certain.

You should stay claim now. The easiest way would be go to the pharmacy and buy a pregnancy test kits, or in deed better to request to talk to a pharmacist, they usually have a private consultation room now and can advise you what you can do.

Usually if it is within 3days post sex, women can obtain emergency contraception in the pharmacy requesting the pharmacist or staff.