My period is 4 days late and i don't know why and i don't know what to do?

Take pregnancy test: Home pregnancy tests are very inexpensive & readily available at most grocery & drug stores. So stop fretting & take test. Once your test returns normal/negative, then know that periods are often irregular at your age. You can regulate your menses to some extent by taking birth control pills which then lower risk of pregnancy if taken properly but don't prevent sexually transmitted diseases.

Pregnancy test: Irregular periods are one of the most common problems obgyns treat. If this is one time problem it usually resolves on its own. If this is a recurring problem you should be evaluated. Infections & hormonal changes can cause spotting. If there is a concern for pregnancy then do a home pregnancy test . There are many options available to help regulate the menstrual cycle. Schedule a visit.