The idea of adding a four-point shot line is a possibility but not one that is likely to happen anytime soon. Two reasons for this might be that the size of the basketball courts and arenas might have to enlarged and refurbished. Also, consider how long it took the NBA to add the three-point shot.
It was first experimented with in the American Basketball Association in 1961 and it took the NBA 18 years or until the 1979-1980 season to implement its use. Before being implemented in the NBA, the addition of a four-point shot would also likely have to be tested for several years, most likely in the D (development) League.