What is the fourth step in the functioning of an M4 Rifle?

The functioning sequence of the M4 Rifle involves several distinct steps that occur when the rifle is fired. The correct answer is firing, which is indeed the fourth step in this sequence.

When the trigger is pulled, the firing pin strikes the primer of the cartridge, igniting the gunpowder and causing the round to discharge. This marks a critical point in the firing cycle, as it is the moment when the rifle actually propels a projectile down the barrel. The firing step is essential to the operation of the weapon, as it leads directly to the subsequent steps of cycling the action and preparing for the next round.

In contrast, extracting refers to the step where the spent cartridge case is removed from the chamber, which occurs after firing. Chambering involves loading a fresh cartridge into the chamber, happening before the firing step. Cocking refers to the mechanism that resets the firing pin after the weapon is fired, which also precedes the firing action. Thus, firing is correctly identified as the pivotal fourth step in the M4 Rifle's operation cycle.