This is the final down a team has to reach the ten yard goal set out for each set of downs. In most scenarios teams will either punt the ball to the opposition or kick a field goal on this down.
This can be the most exciting down in football. If you are unable to convert on fourth down the opposition gets possession of the football wherever you were on the field. In a 2009 game between the Colts and the Patriots Bill Belichik decided to go for it on fourth and inches in his own end of the field. The Patriots were unable to convert and the Colts went on to win the game.