Gameplay
Call of Duty 4: Modern Warfare is a first-person shooter similar to previous games in the Call of Duty series but set in the present-day instead of World War II.

The game's move to modern warfare introduces new weapons and technology to the Call of Duty franchise, including the M4A1 carbine, the M203 grenade launcher, the AN/PEQ-2 Target Pointer for use in conjunction with night vision goggles, the MP5SD submachine gun, and the FGM-148 Javelin portable anti-tank guided missile. The player gains access to these over the course of the game, but may only carry up to two weapons in addition to grenades. Weapons from fallen foes can be picked up to replace weapons in a player's arsenal. Players also have additional abilities, including a grenade launcher attachment, claymores with tripwire-like detonation (appears as a V-shaped motion sensor laser in-game), C-4, night-vision goggles, and the ability to call in airstrikes and an attack helicopter.

The gameplay of Call of Duty 4 shares several features with previous iterations of the franchise. Once again, players fight alongside AI-controlled teammates. They help during the game's missions by providing cover fire, shooting down enemies, and clearing rooms for entry.

A character can be positioned in one of three stances: standing, kneeling, or prone; each affecting the character's rate of movement, accuracy, and stealth. Using cover helps the player avoid enemy fire or recover health after taking significant damage, as there are no armor or health powerups. When the character has taken damage, the edges of the screen glow red and the character's heartbeat increases. If the character stays out of fire, the character can recover. When the character is within the blast radius of a live grenade, a marker indicates the direction of the grenade, helping the player to either flee or toss it back to the enemy.


Campaign
Similar to previous games in the series, the player takes on the role of various characters during a single-player campaign. The characters' involvement in the plot occurs simultaneously and overlaps the events in the game. As such, the player's perspective can change from one character to another between missions.

Each mission features a series of objectives; the player is led to each objective with the HUD, which marks its direction and distance. Some objectives require that the player arrives at a checkpoint, while other objectives require the player to eliminate enemies in a specified location, stand their ground to defend an objective, or plant explosive charges on an enemy installation. The player is often accompanied by friendly troops, who cannot be issued orders. Laptop computers containing enemy intelligence appear throughout the campaign, and can be collected to unlock game bonuses.

After the credits roll, a special epilogue mission is unlocked for play, featuring an anonymous four-man squad of SAS troops retrieving a VIP from terrorists who have hijacked an airliner. The SAS rescue the VIP and escape before the plane is destroyed.


Multiplayer
Call of Duty 4 features team-based and deathmatch-based multiplayer modes on various maps. Each mode has an objective that requires unique strategies to complete. Players can call in UAV reconnaissance scans, air strikes, and attack helicopters, either an AH-1 Cobra for S.A.S. or USMC or an Mil Mi-24 for OpFor or Spetsnaz, when they achieve three-, five-, and seven-enemy kill streaks respectively. A game ends when either a team has reached a predefined number of points, or the allotted time expires in which case the team with the most points wins. However in the rare case the points are even when the time expires, Sudden Death mode is activated in which there is no re-spawning and the team who either has the last man standing, or achieves the objective first are the winners.

The player's performance in the multiplayer mode is tracked with experience points, earned by taking down other players, completing objectives, or by being a member of a winning team. As the player gains experience, they advance in level, unlocking new weapons, perks, challenges, and gameplay modes. The highest obtainable level is 55, but on the console versions of the game, the player has the option to play "Prestige" mode; this resets their level back to 1 and all earned bonuses are lost in exchange for a special in-game insignia. This process can be repeated up to 10 times with a different insignia being given each time, giving the player a total of 604 levels to achieve.

Completing a challenge grants experience points and may unlock weapon attachments. When a player's level increases, it may unlock new weapons, perks, or challenges. At lower experience levels, the player only has access to five pre-determined classes, and cannot create a custom class with the weapon of their choosing. However, as the player advances in levels, they earn the ability to customize their classes. This includes selecting their main weapon, side arm, and special grenade type. Additionally, the player can select a limited number of "perks" that can customize their character further. Perk effects include increasing damage by the player, being able to take more damage, or dropping a live grenade after being killed in an act of martyrdom. The player is also given challenges to attempt, including achieving a certain number of kills with a specific weapon or shooting down a helicopter or performing a number of head shots.


Characters
During the single-player campaign, the player controls six different characters from a first-person perspective. The player assumes the role of "Soap" MacTavish for most of the game, starting with his enrollment in 22nd SAS. Sergeant Paul Jackson is part of USMC 1st Force Recon deployed to the Middle East, and the player controls Jackson's character during five levels of Act 1. Captain/Lieutenant Price is an officer of 22 SAS who is playable in a flashback. Price is voiced by actor Billy Murray, most famous for playing Don Beech on the television series The Bill. Yasir Al-Fulani is the president of the unnamed Middle Eastern country mentioned in the game, and is playable only in the game's opening credit sequence before he is executed. The player assumes the role of an American thermal-imaging TV operator aboard an AC-130 gunship during one level, and a British SAS counter-terrorist operative infiltrating a hijacked airliner to save a VIP in the epilogue level.

The game's non-playable characters (NPC) feature prominently in the story. Captain Price (in his NPC capacity) and his right-hand man, Gaz (voiced by Craig Fairbrass), serve as mentors to MacTavish. Jackson's USMC platoon is led by Lieutenant Vasquez (voiced by David Sobolov) and Staff Sergeant Griggs (voiced by and modeled after Infinity Ward lead animator Mark Grigsby); Griggs later accompanies MacTavish in Russia. Sergeant Kamarov leads the Russian Loyalists that ally with the SAS and USMC forces. Nikolai is a Russian informant who helps the SAS. Captain Macmillan is Price's (then a lieutenant) mentor and commanding officer during the flashback to the assassination attempt on Zakhaev. The villains in the story are Imran Zakhaev, the leader of the Russian Ultranationalist party and the main antagonist of the game; Khaled Al-Asad, the commander of the revolutionary forces in the Middle East and an ally of Imran Zakhaev; and Victor Zakhaev, the son of Imran Zakhaev and a priority figure in the Ultranationalist party.


Plot
On a mission in the Bering Sea, Sergeant "Soap" MacTavish, Captain Price, "Gaz", and several SAS members find a nuclear device onboard a cargo ship. Suddenly, the ship is attacked, and the team evacuate with the cargo manifest, which provides evidence of ties between the Russian Ultranationalist Party and a rebel faction in the Middle East. Russian Ultranationalist Imran Zakhaev, who plans to return his homeland to the times of the Soviet Union, draws international attention away from his plans by funding a coup d'état in an unnamed Middle Eastern country, organized by a local separatist leader named Khaled Al-Asad. The British and American governments, who discover the plot, initiate a police action to stop the uprisings in both regions. After President Al-Fulani of the Middle Eastern country is executed on live television and Al-Asad takes control, the SAS rescue their compromised informant in the Russian Ultranationalist Party, Nikolai.

In an American invasion of the Middle Eastern country, a platoon from the USMC 1st Force Recon, led by Lieutenant Vasquez and Sergeant Paul Jackson, searches for Al-Asad, but only manages to secure a television station broadcasting Al-Asad's propaganda. During the final stages of the operation, United States Central Command is notified by Seal Team Six of a Russian nuclear weapon nearby, and sends the NEST to disarm it. However, the nuclear device suddenly detonates, leveling most of the city and killing everyone in the area, including Vasquez's squad, whose helicopter had not left the blast radius after coming to the last-minute rescue of the pilot of a downed AH-1 Cobra helicopter.

The British learn that Al-Asad fled the country before the USMC's invasion. With the help of their informant Nikolai, the SAS finds and interrogates Al-Asad. After learning from him that Zakhaev supplied the nuclear bomb, Captain Price executes Al-Asad. Price then has a flashback of his mission to eliminate Zakhaev in Prypiat, Ukraine, 15 years earlier. In the aftermath of the Chernobyl disaster and the collapse of the Soviet Union, Zakhaev took advantage of the turmoil to profit from nuclear proliferation and used his new wealth to lure soldiers from the Soviet Army to form his Ultranationalist Party. Price was paired with Captain MacMillan to carry out the black op assassination of Zakhaev. After stealthily sneaking into Prypiat and taking up a position in the top room of an abandoned hotel, MacMillan spotted Zakhaev while Price fired a .50 BMG sniper rifle, but Zakhaev survived, losing only an arm. Price and MacMillan were pursued from the scene and narrowly escaped.

Back in the present, a joint operation, by the SAS regiment under the command of Captain Price, a USMC Force Recon unit led by Staff Sergeant Griggs, and forces from the Russian military led by Sgt. Kamarov, is undertaken to stop Zakhaev. They capture his son Victor to learn of Zakhaev's whereabouts, but before they can question him, Zakhaev's son commits suicide. Zakhaev becomes enraged, blaming Western nations for the death of his son, and plans to retaliate by launching ballistic missiles armed with warheads at the United States. When SAS/USMC operatives arrive at the facility, Zakhaev manages to launch two missiles towards the United States. However, the squad successfully deactivates the missiles and escapes the facility in military trucks, Zakhaev's forces in hot pursuit.

Before the squad can escape across a nearby bridge, it is destroyed by a gunship, leaving them trapped. Zakhaev's forces arrive and engage the remaining members of the strike force. Suddenly, Gaz receives a call from Sergeant Kamarov, informing him that his forces are on their way to help the American and British forces. On the bridge, a gas tanker explodes, incapacitating most soldiers nearby except Griggs, who is shot while trying to pull MacTavish to safety. Zakhaev, along with two of his soldiers, finish off Gaz and other surviving members of the strike force. He then begins to walk towards Soap and Price, but is distracted by the destruction of his gunship and the arrival of a Russian military helicopter. As Zakhaev looks away, Price slides his pistol to Soap, who shoots and kills Zakhaev and his two guards. When Kamarov and his team arrive, MacTavish is evacuated to safety, while a Russian medic attempts to resuscitate Price. MacTavish survives and becomes a captain in the SAS before leading an elite military force in Modern Warfare 2, but Price's ultimate fate is unknown.


Development
Call of Duty 4 was developed by a team of a hundred people, over the course of two years. After Call of Duty 2, the Infinity Ward team decided to move away from the World War II environment of previous games in the series. This resulted in two game concepts: Call of Duty 4: Modern Warfare and an unnamed game set to be released in the future. While developing the story for Call of Duty 4, Infinity Ward chose to avoid referencing current, real-life wars, and keep the series' common theme of two opposing forces of similar strength. To enhance the realistic feel of the game, the development team attended a live-fire exercise at Marine Corps Air Ground Combat Center Twentynine Palms, a training facility in the California desert. This helped the developers to simulate the effects of being near an Abrams tank when it fires. The team also talked with United States Marines who were recently in combat to get a feel for the background, emotions, and attitude of Marines in combat. Veterans were also recruited to supervise motion capture sessions and the artificial intelligence design of the game.

The development team designed the online multiplayer component to be balanced and rewarding for new players while still offering something for experienced players. An early idea to implement air support (air strikes and attack helicopters) involved players fighting over special zones to access a trigger for air support against enemies. This idea was discarded because it discouraged the type of deathmatch gameplay they intended. The kill streak reward system was put in its place to encourage the improvement of player skills. Players were allowed to select weapons before matches to get accustomed to weapons more easily and minimize weapon hunting. Maps were designed primarily for deathmatch games—the developers felt such designs suited other types of gameplay as well. Map layouts were designed to minimize locations players could hide from enemy gunfire.


Audio
Most of the music for Call of Duty 4: Modern Warfare was written by British composer Stephen Barton, who had also contributed to film scores by Harry Gregson-Williams. Gregson-Williams also composed music for the game, such as the main theme. Several music tracks from the game are available on Infinity Ward's "7 Days of Modern Warfare" website, and some are available at Barton's own web site. The rap song played during the end credits is performed by Call of Duty 4's lead animator, Mark Grigsby.


Game engine
Call of Duty 4: Modern Warfare runs on a proprietary engine with features that include true world-dynamic lighting, HDR lighting effects, dynamic shadows and depth of field. "Bullet Penetration" is calculated by the engine, taking into account factors such as surface type and entity thickness. The game runs in a native resolution of 600p for its console versions.

Certain objects, such as cars and some buildings, are destructible. This makes distinguishing cover from concealment important, as the protection provided by objects such as wooden fences and thin walls do not completely protect players from harm. Bullet speed and stopping power are decreased after penetrating an object, and the decrease is dependent on the thickness and surface type of the object. The game makes use of a dynamic physics engine, not implemented in previous Call of Duty titles. Death animations are a combination of pre-set animations and ragdoll physics. Console versions of Call of Duty 4: Modern Warfare run at a consistent 60 frames per second. Code was included to determine spawning points based on the nearby weapons, and enemy positions and line of sight. The various criteria are meant to minimize players dying immediately after rejoining a match, or being "spawn-killed" due to players simply waiting for players to "respawn".

The game engine has also been used for the development of two other Activision games. An enhanced version of the original engine was used in Call of Duty: World at War, the fifth installment in the Call of Duty series after Call of Duty 4, while a slightly altered version has been used for the James Bond video game Quantum of Solace.