Grand Theft Auto IV is structured similarly to previous games in the series. The core gameplay consists of elements of a third-person shooter and a driving game, affording the player a large, open environment in which to move around. On foot, the player's character is capable of walking, running, swimming, climbing and jumping, as well as utilising weapons and basic hand-to-hand combat. Players can steal and drive a variety of vehicles, including automobiles, boats, helicopters, and motorcycles. Grand Theft Auto IV takes advantage of Natural Motion's Euphoria engine, which combines artificial intelligence, bio mechanics and physics to make NPC behaviour and movement adaptive and more realistic.

The open, non-linear environment allows players to explore and choose how they wish to play the game. Although storyline missions are necessary to progress through the game and unlock certain parts of the map and content, they are not required, as players can complete them at their own leisure. When not taking on a storyline mission, players can free-roam. However, creating havoc can attract unwanted and potentially fatal attention from the authorities.

It is possible to have multiple active missions, due to the fact that some missions will run over the course of several days and will require the player to wait for further instructions, etc. The player can also partake in a variety of optional side missions. For the first time in the series, Grand Theft Auto IV features "morality choices" at several points throughout the game, in which the player is forced to choose between killing a character or sparing their life or killing one of two characters. The game has two different endings, which are determined by deciding which of the two missions to complete.The player can choose between a revenge mission or a deal mission each leads to a different ending.

Multiplayer

Grand Theft Auto IV also includes online multiplayer, with 15 modes of play available.The multiplayer supports up to 16 players and allows players to explore the entire city.Players use a customisable character in a majority of the modes, and cash earned during play translates to a level at which more clothing is available for their character, depending on the level. Hosts of the games can control many variables, such as time of day, police presence, pedestrian presence, traffic, location of game, and weapons. The online games are split into ranked and unranked matches. The reward for the ranked gameplay is cash, which determines players' ranks.The game does not feature any split screen or LAN multiplayer modes.

There are several different game modes available. Team based gameplay modes include Team Deathmatch, where 2-8 teams compete to accumulate the most kills in a traditional deathmatch; Team Mafiya Work, in which 2–8 teams compete to complete contract work for the "mafiya", such as escorting/killing targets or stealing cars; Team Car Jack City where 2–8 teams compete to steal cars and earn money for keeping them undamaged; Cops n' Crooks, featuring a team of cops who must compete against a team of crooks (which features the "All for One" variation - requiring the cops to kill the crooks' "Boss" before he is escorted to the extraction point - and the "One for All" variation - requiring the cops to kill all of the crooks before they reach the extraction point); and Turf War, involving two teams who compete to take control of designated areas of the map and control them for as long as possible.

The game also includes a variety of racing and cooperative modes, which include Race, in which players race through checkpoints in a traditional automobile race; a GTA Race variation, where players race through checkpoints in an automobile race, with the ability to combat their opponents; Hangman's N.O.O.S.E., a co-op mode that requires players to collect a person from the airport and safely escort him to the extraction point before the cops kill him; Deal Breaker, a co-op mission that requires players to assault a construction site captured by enemies, then chase a group of enemies before they escape; and Bomb da Base II, a co-op mission that requires players to clear out a ship, then destroy it with explosives. 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.

The PC version supports 32 players in the game's various online multiplayer modes.