The Four Horsemen

The Four Horsemen is the name for a group of four men whose aims appear to become the dominant world power and usurp the Russian Federation. They are named after the Four Horsemen of the Apocalypse. It is the player's objective throughout Call of Duty 4: Modern Warfare to uncover information on them and eventually eliminate three of the four. They all have some form of command role in their respective groups. Achievements are associated with them, as the player gets an achievement each time they complete a mission on Veteran that a Horseman dies in, the only exception being The First Horseman, Vladimir Makarov.

The First Horseman
The First Horseman is an unidentified man who was killed some time before the mission Safehouse.

The Second Horseman
The Second Horseman is Khaled Al-Asad, the leader of an unnamed Middle-Eastern nation following a bloody coup. He is an ally of Imran Zakhaev. It was believed that he was the one responsible for the detonation of the nuclear bomb in the capital, killing many of the invading U.S. forces (30,000, as Shepherd later revealed), but was later revealed to be Makarov that detonated it. He is later captured alive in his safehouse and executed by Captain Price in the twelfth mission "Safehouse". Zakhaev later deployed a large number of Ultranationalists to rescue him, but it was too late. Beating this mission on any difficulty unlocks "Wrong Number", while beating it on Veteran awards "The Second Horseman".

The Third Horseman
The Third Horseman is Victor Zakhaev, the son of Imran Zakhaev. Victor is the commander of the Ultranationalist military forces on the ground in Russia. He committed suicide before he could be caught in the sixteenth mission, "The Sins of the Father". The achievement "Desperate Measures" is achieved simply for beating Sins of the Father on any difficulty, while "The Third Horseman" is achieved for beating "The Sins of the Father", and the previous mission "Heat", on Veteran.

The Fourth Horseman
The Fourth Horseman is Imran Zakhaev, a Russian Ultranationalist and leader of the "Horsemen". He supplies Al-Asad with weapons, including a nuclear weapon. He was thought to be dead 15 years before the main events of the game, having been shot by Lieutenant Price and Captain MacMillan. However, he survives this encounter (being saved by Makarov and Yuri) and "only" loses his left arm. He is killed by Sergeant John "Soap" MacTavish in the final mission "Game Over". The Achievement "The Fourth Horseman" is awarded for completing said mission on Veteran.

Trivia

 * The Four Horsemen are a reference to the biblical Four Horsemen of the Apocalypse.
 * The three Horsemen present in Call of Duty 4 are all killed by an M1911.
 * Titles of three of the four horsemen can be found in Modern Warfare 2's files.
 * There are achievements associated with completing the missions in which each is killed, with the exception of the First Horseman, as Makarov was not present in the first game.
 * Captain Price has witnessed the deaths of all the horsemen and has personally killed two of them (Khaled Al-Asad and Vladimir Makarov).
 * Price and Soap are the only protagonists to have seen all four Horsemen, but Price is the only one to have seen all four up close.
 * Until Modern Warfare 3 confirmed it was Makarov, the identity of the First Horseman was unknown, as all references to the group as a whole began with the second, Al-Asad, and the photo of the four together already had the first's face crossed out.