The NBA key has 4 hash marks on each side. Three are required to separate the three players during a free throw. The fourth is situated close to the third. What is the fourth one for?
There is no stated purpose for requiring four hash marks along the free throw key. In fact, it is up to the arena's discretion for where exactly they are placed. The only requirement involves how far away the hash marks are from the foul line and the baseline.
The NBA rule book states:
Four hash marks shall be drawn (2” wide) parallel to the baseline on each side of the free throw circle. These hash marks shall be 13’ from the baseline and 3’ from the free throw lane lines and shall be 6” in length.