Girlfriends late on her period?

ok my girlfriend is begining to get worried about possibly being pregnant. she is upset because she and her sister normally start their periods within a day or so. but she is about 3-4 days late. Is it normal to be a little late every once in a while and alo what are the chances of her being pregnant, i mean only once have we not used a condom and i pulled out, and that was after we had already had that day. So i quess what i want to ask, is it normal to be a few days late, what are the chances she is pregnant, and how long do i have to wait before i get a pregnancy test.