If your on birth control, then you should not be pregnant. If you stop taking the pill, your period will come through early. If you suspect your pregnant, then stop taking the pill.
If you really want to stop them, you can, but be sure to wait the 7 days before starting the new pack. If I were you, I'd probably just take the four pills and then see since it's four more days of protection if you're not trying to get pregnant and if you are pregnant, the baby's not far enough along to really be effected.