Sometimes not eating right, stress, or hormone fluctuations can cause you to miss periods. It's not unusual to miss one period here or there due to many different reasons, and if you've done four tests it's definitely not pregnancy or you could be testing at a time when your hcg level (what determines pregnancy in tests) is low enough to show that you wouldn't be pregnant. If you test first thing in the morning when you wake up and it is negative you more than likely aren't.