If your period is 4 days late, there are several possible reasons for this delay. Sometimes, a sudden illness, stress, fatigue, or changes in your routine can all cause disruptions to your menstrual cycle. In most cases, the situation tends to resolve itself without any further issues. However, if you have had unprotected sex since your last period, it may be a good idea to take a pregnancy test after 3 to 5 days or until your next period arrives. It's always best to consult with a healthcare professional if you have any concerns or questions regarding your menstrual cycle.