Four Horsemen

The Four Horsemen (の) is a group of four powerful Devils: the Control Devil, War Devil, Famine Devil and Death Devil.

Overview
The Four Horsemen are the only individuals who still remember the names of Devils which have been devoured by the Chainsaw Man, such as the Nuclear Weapons Devil, the Nazi Devil, and others. However, it is unclear how far these memories extend: Makima, one incarnation of the Control Devil, could remember the names of devoured Devils but said that even her memories were starting to fade. The next incarnation, Nayuta, appears to bear no memories from her predecessor’s life but this is not enough evidence to determine if she automatically remembers the names of the erased or not.

Despite collaborating together, the Four Horsemen do not appear to be a cohesive unit, with each member having different goals. Makima, a former member of the Four Horsemen, intended to use Pochita's power to eradicate her fellow Horsemen despite previously battling alongside them against Pochita, whilst the War Devil wants to avoid the Famine Devil and its main goal is to make Pochita throw up the Nuclear Weapon Devil and revive him. Meanwhile, Famine is involved in the apocalypse prophesized by Nostradamus and is currently trying to prevent it from happening and enslave her sister War to aid her in this goal. Nayuta on the other hand doesn't care to involve herself with her sisters and would rather study.

It is also implied that the combined members of the Four Horsemen are almost as powerful, if not on par, with the Primal Fears due to Makima being able to slightly wound the Darkness Devil, albeit at the cost of her arm, and Fami being able to enslave the Falling Devil. However, the Horsemen do not command the same fear and respect as the Primal Fears; the reincarnated Eternity Devil openly cursed and insulted Fami when their deal didn't go according to plan.

Appearance
Though the members of the Four Horsemen vary in individual appearance and can look different upon reincarnation, additionally, they are all shown to be female. While only two known incarnations of the Control Devil have been humanoids the true forms of the rest are unknown or debatable at best, the War Devil has an unknown true form, with the bird appearance being a near death state, but possesses the body of a teenage girl. The nature of Famine Devil’s appearance is also debatable as the War Devil did not recognize her. The Death Devil has an unknown appearance.

Eyes
Each of the Horsemen is shown to have ringed irises of their eyes. Additionally each Horsemen appears to have their own unique eye color.

History
The Four Horsemen and the Weapon Devils once engaged in a brutal battle against Pochita in Hell. However, in the midst of the battle, Pochita mysteriously vanished. Despite searches by the Control Devil, Pochita could not be found. Unbeknownst to the Four Horsemen, this was because Pochita had changed forms on the verge of death and escaped to Earth. This is where he and Denji would first meet.

Sometime after this battle, the Horsemen arrived on Earth, but they part ways. The Control Devil assumed the identity of a human, Makima, who became the head of Special Division 4 in the Public Safety. The War Devil took the form of an owl before possessing Asa Mitaka, where she was later renamed as Yoru. The Famine Devil (nicknaming herself Fami) assumed an identity of a high school student in Fourth East, being a member of the school's Devil Hunter Club. The Death Devil's whereabouts remained unknown.

Control Devil arc
Makima reveals the existence of the Four Horsemen, describing how she, the Horsemen, and the Weapon Devils fought against Pochita before the battle ended after Pochita suddenly vanished. With the Weapon Hybrids present, she announces that the interrupted battle can finally be brought to an end, as she orders the Weapon Hybrids to attack Chainsaw Man.

After seemingly succeeding at defeating Chainsaw Man and seizing control over him, Makima is suddenly sliced open by Denji with a chainsaw. Through a loophole, Denji renders the Devil Contract Makima had with the Prime Minister of Japan that gave her nigh-immortality null and kills her by eating her entire body.

The Control Devil is soon reincarnated as Nayuta, a young girl from China. Kishibe discovers her and has her brought back to Japan, where he discharges her into the care of Denji, believing that Denji is the best option for caring for Nayuta.

Justice Devil arc
The War Devil, in an owl-like form, appears in front of Asa Mitaka shortly after she is murdered by her jealous classmate, who murdered her out of the belief that it would secure the affections of their teacher Mr. Tanaka. The War Devil offers to save her life, in exchange for control over her body. Mitaka takes this opportunity, allowing the War Devil to assume control. The War Devil immediately slaughters her murderer as well as her teacher. Seeing a Chainsaw Man poster nearby the slaughter, the War Devil vows to make Chainsaw Man "vomit nuclear weapons back up".

Falling Devil arc
Yoshida reveals the "Prophecy of Nostradamus" which foresees a coming apocalypse. Their involvement in said prophecy varies from member to member; with Famine actively working against it, Control seems disinterested and Wars & Deaths stance is currently unknown.

Yoru correctly guessed that Fami is involved with the current Primal Fear attack that's related to the aforementioned prophecy. It was later revealed by Fami she was trying to trap Yoru in a devils belly to starve her and turn her into her pawn, as she can control those who're starved. Nayuta declined the offer to work with Fami.

Trivia

 * Before being revealed to be a member of the Four Horsemen, Makima was highly implied to be a member beforehand due to her and the Conquest Horsemen sharing the same name in Japanese, which is domination.
 * Despite this, it was Makima's stated goal to use the Chainsaw Devil get rid of the other three Horsemen: Death, War, and Hunger.
 * The Four Horsemen of the Apocalypse are figures in the Christian mythology, first appearing in the Old Testament's prophetic Book of Zechariah and in the Book of Ezekiel, where they are named as punishments from God: Conquest, War, Hunger, and Death.
 * The order of the Horsemen are also reflecting the order of their respective Devils shown in the series: first Conquest, followed by War then Famine.
 * Under another interpretation, the first Horseman is called Pestilence, and is associated with infectious disease and plague. The reason for this was because scholars found Conquest and War to be too similar so an easy change was to make Conquest into Pestilence as the white horseman would carry a bow and arrow and bows and arrows were associated with disease at the time as they would often be laced with toxins and poisons.
 * While Makima can control any creature, including any animals, she is shown to teleport herself with rats which are creatures often associated with pestilence.
 * Their eye shapes seem to be based off of Nayuta from Tatsuki Fujimoto's one-shot Yogen No Nayuta.
 * Of course, the first Horseman’s reincarnation is obviously based on her.
 * Nayuta was a demon child prophesized to start the apocalypse which is similar to how the common full title of the four horsemen is “The Four Horsemen of the Apocalypse” in media.
 * Just because a Devil has ringed eyes does not make them part of the Four Horsemen (whether it extends towards the sclera or not). The Fox Devil and Future Devil being prime examples.
 * So far, three of the Four Horsemen showcased to possess an ability and power that allows them to take control and ownership of something or someone they believe is “theirs” based on their own perception and opinion. It is currently unknown if this applies to all horsemen as the full scale of Death's powers are unknown.
 * Control has demonstrated the power to command and brainwash anyone she deems “inferior” to her and anyone who believes themselves inferior to her. Allowing her to control animals, humans, devils, fiends and even hybrids.
 * War has showcased the ability to transform anything and anyone she believes belongs to her into a weapon. As shown with Asa’s usage of this ability, War’s ability is based completely on their perception regardless of the common sense or logic behind the reason for their ownership over that something or someone. (Ex. Gaining ownership over an Aquarium by “buying” it despite not having enough money to actually pay for it, nor gaining the actual owners’ legal consent to the transfer of its ownership)
 * Fami has also demonstrated and stated to have the the ability to turn anyone who is “starved” into her servants. Its been implied that this ability works regardless of how powerful the target is, as shown where Fami has shown to have successfully subjugated a Primal Fear into her service. An additional part of this ability is the power to transform said servants into literal pawns,  as shown where she transformed the Falling Devil into a miniature statue for her to transport them more easily.
 * While they can’t kill the unkillable Primal Fears, judging from the interaction and battle between the Control Devil and Darkness Devil, they at least seem to be feared enough by some them because they can still severely injure them and give them immense pain while other lower ranking Devils won’t even stand a chance in a battle. This fear towards the Four Horsemen is also possibly due to the Four Horsemen’s unique abilities and powers capable of affecting the Primal Fears, as shown with the Falling Devil and the Famine Devil, where Fami was able to successfully subjugate the Primal Fear and turn her into her servant.
 * It is unknown for how long the Horseman have been fighting Pochita, but Yoru mentions that there hasn't been a war after WW2 ever since she got partially eaten and weakened by Chainsaw Man, suggesting that the Horsemen have fighting him for over 50 years.