Boundary 4

A boundary 4 is a ball that touches or rolls along the ground at some point before finally reaching or crossing the boundary.

It may have been struck by the bat or be a No ball, Wide or Bye that has travelled that far. It scores 4 runs, unless there is any pre-match agreement between the umpires and the captains that some other allowance should apply (this may be because of a local convention).
It is called a boundary 4 to distinguish it from occasions when the striker might score 4 runs by running them consecutively - the ball not having crossed the boundary at all. This can happen quite frequently on grounds with a large field of play and fit batsmen running.
Very occasionally it may happen that the batsmen complete more than 4 runs before the ball crosses the boundary and they should be credited with the higher total. They may have run 4 and already crossed on their fifth, for instance. If they crossed before the ball reached the boundary then that fifth run will count, too, and the umpires may need to ensure that the scorers understand that 5 runs are to be scored.