In the game of cricket, four runs are awarded to a batter who hits a ball all the way to the boundary. Such a shot is referred to simply as a "four". If the ball goes over the boundary on the full, six runs are awarded and the shot is referred to as a "six". These shots are roughly analogous to a home run in baseball, however there is no requirement for the batter to actually run as there is in a baseball home run. It is common to see a batter hit a four, and then remain standing serenely at the crease while the fielding team fetch and return the ball.
In each batter's innings, the number of fours is recorded, and gives an indication of the how the innings was played. A century which contains 20 fours indicates that the batter was in a particularly aggressive mood. If it contained only 3 fours it would indicate an unusually defensive innings (or perhaps a very slow outfield).