I have been on birth control for about four weeks. I just had sex last night and he ejaculated inside me. Could I be pregnant?

Once you have been on it for one month, you can generally consider that you will be protected. How close is "about four weeks"? Even if it was slightly less, you should be OK.

Of course you have to make sure you follow directions for it carefully... ideally taking the pill the same time each day and not missing any.

There will always be a slight chance of pregnancy even when birth control is used correctly, but odds are you will be fine.