) If you would like, keep a chart on graph paper of the days of your
menstrual months (or use an online
chart) -- sometimes women assume their period is supposed to come on a certain calendar day without remembering that it is supposed to come on a certain day based on the first day of the previous period (such as, every 28th day or every 29th day from the last period).
If you've got all that right and are sure you're 4 days late, just wait and see.