The result from a pregnancy test, that is made 4 days after your period missing, is valid so, you are probably not pregnant. That is understandable if we have in mind that birth control efficiency is over 99%. A period occurs when hormone levels (estrogen and progesterone) drop. When using birth control pills, a period occurs when you start using the last 7 inactive pills (in 28 pills pack) or make a 7 days pause (in 21 pills pack). Sometimes cycle/period irregularities are possible even when using birth control pills but there is nothing to worry about. Several things can disturb the hormonal balance including stress. Presence of functionally active ovarian cysts can also cause such cycle irregularity. In that case, a period can be delayed as long as the cyst persists (a few months usually).
You can do another pregnancy test for sure and, if it is still negative, you can continue taking the pills. If a period is delayed more than 2 months, you may want to consult your gynecologist.
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