6 Answers

Stress could be making it late. Also if you are not completely regular.

Best to buy a pregnancy kit. Then go see your doctor and maybe some birth protection if you don't want a baby right now.

Four days is not unusual.  I would follow the directions of Dardaigh.  It is best not to panic.

If you are the first thing you want to do is make sure it yours with all the false claims going on now our days , a paternity test would be in order....ok seriously 4 days late is really not such a big deal , just to be safe why not get a ept or see a doctor and next time be prepared hopefully this is just a wake up call..

Colleen

If she is pregnant, how could it not be hers?

Not by me. Did you do anything to put yourself in a position to get preg, accidentally or not?

Do you keep a calendar and make a note of the first day of EVERY period?  If so, and your record consistently shows the same range of days from start to start, it's possible.  Of course, being sexually active increases the likelihood.  
Do what has been suggested.  Get a home pregnancy test and find out.  You aren't going to really get an answer to trust here.  No one has a crystal ball (of which I am aware).  

Wait about nine month! Then you'll see if you are ?