Grand Theft Auto IV is an action-adventure game played from a third-person perspective. Players complete missions—linear scenarios with set objectives—to progress through the story. It is possible to have several active missions running at one time, as some require players to wait for further instructions or events. Outside of missions, players can freely roam the game's open world and complete optional side missions. Composed of the fictional city of Liberty City, the world is larger in area than most earlier Grand Theft Auto series entries. At the beginning of the game, players can only explore the first island—composed of Dukes and Broker—with all other islands unlocking as the story progresses.
Players use melee attacks, firearms and explosives to fight enemies, and may run, jump, swim or use vehicles to navigate the game's world. There is a first-person perspective option when using vehicles. In combat, auto-aim and a cover system can be used as assistance against enemies. Should players take damage, their health meter can be fully regenerated by eating, using medical kits, or calling for paramedics. If players commit crimes, the game's law enforcement agencies may respond as indicated by a "wanted" meter in the head-up display (HUD). On the meter, the displayed stars indicate the current wanted level (for example, at the maximum six-star level, efforts by law enforcement to incapacitate players become very aggressive). Law enforcement officers will search for players who leave the wanted vicinity. The wanted meter enters a cool-down mode and eventually recedes when players are hidden from the officers' line of sight.The game's cover system allows players to move between cover, to fire blindly, aim freely, and target a specific enemy. Individual body parts can also be targeted. Melee attacks include additional moves, such as dodging, blocking, disarming an opponent and counter-attacking. Body armour can be used to absorb gunshots and explosive damage, but is used up in the process. When health is entirely depleted, gameplay stops, and players respawn at the nearest hospital.
The single-player mode lets players control an Eastern European war veteran, Niko Bellic. During the story, Niko meets and befriends various new characters. They can then perform favours for Niko whenever he asks; for example, his cousin Roman, who owns a taxi service, can send one of his cabs to take Niko to any destination around the city. Cabs are always available during gameplay for quick travel to a destination. Throughout the course of the game, players are also faced with morality choices, which alter the storyline appropriately depending on the player's choice. While free roaming the game world, players may engage in context-specific activities such as bowling or darts. Other available activities include a vigilante mini-game, and in-game television programming. Niko has a cell phone for contacting friends and starting activities. The cell phone is also used to access the game's online multiplayer mode, and to enter cheat codes. To access the in-game Internet, which allows Niko to send and receive emails and set up prospective dates with potential girlfriends, Niko can use Internet cafés located around the city. The game also features a subway system, allowing players to quickly traverse through the game's world.
The online multiplayer mode for Grand Theft Auto IV allows up to 32 players[b] to freely roam across the map. Players decide which game mode they wish to play, including deathmatches and street races. Both cooperative and competitive game modes are available, split into ranked and unranked matches. For players to level up through ranks, in-game money has to be earned. The game also features a Free Mode, in which players have the entire map open to explore, with no end goal or mission to complete. Hosts of the game can control many variables, such as police presence, traffic, and weapons. The multiplayer mode was discontinued on Windows in 2020.