Used for a projectile that has the seeker missile thing type flag set and is fired from a codepointer that allows the missile to be homing. When this codepointer is triggered at each fourth tic, it will cause the projectile to change direction towards its shooter's target, as well as spawn smoke.
Note that by how the projectile looping states are set up, they may have even durations, causing the current tic to never be a multiple of four, such as: 3, 7, 11, 15... This is the case with Doom 2's revenant missile, and if that happens, it will not home. Using tics of odd durations, however, you can control the turning speed of the projectile, as they will always hit multiples of four.