Four Heavenly Kings

The Four Heavenly Kings (四天王, Shitennō) is the collective term referring to the four individuals that possess the greatest influence within Rukongai. Each of the four Kings generally retain their greatest influence in one of Rukongai's four cardinal district networks, though because they also have influence spread all over, conflict between them is be expected. Furthermore, it is considered an incredibly rare event when two or more of the Kings gather together in a single location; an occasion that puts the Gotei 13 on high alert should the word ever get out on the gathering.

Purpose
While the four generally do not get along very well, they have agreed to a "non-aggression pact" in which they do not engage the other in hostile opposition but rather attempt to settle whatever disputes they may have with one another through discussion and compromise. This is born out of a necessity to maintain their influence without prompting their operations from being discovered by the Gotei 13.

Behind the Scenes
In Buddhism, the Four Heavenly Kings are four deities that watch over the four cardinal directions of the world.