About Call Of Duty 4

A Brief History
Call of Duty 4: Modern Warfare introduces a variety of game play styles into the single player campaign completely new to the Call of Duty series. The game moves away from the standard three country-specific campaign style, and allows the plot to play through more like a film style plot with interlaced story lines from the perspective of the player as Paul Jackson, a member of the United States Marine Corps 1st Force Recon and Soap MacTavish, a member of the British 22 SAS Regiment. There is also a variety of cameo-style missions where you play as various other soldiers such as an AC-130 Spectre Gunship gunner, or as one part of a two-man sniper team behind enemy lines in a flashback-style mission.

Call of Duty 4: Modern Warfare focuses on the variation of modern warfare as an infantry soldier by allowing the player to control soldiers in a vast number of different scenarios, straying away from the constant "at eleven" action of previous Call of Duty titles and allowing for both heavy intensity fire fights and slower paced drama styled intensity. The series' move to modern warfare has also introduced a variety of new modern weapons and technology to be introduced to the Call of Duty franchise. Such showcased weapons and attachments include M203 grenade launchers, and the AN/PEQ-2 Target Pointer for use in conjunction with night vision goggles, silenced MP5SD submachine guns, and the FGM-148 Javelin portable anti-tank guided missile among many others.

Multiplayer Aspect Of Call Of Duty 4
The multiplayer mode in Call of Duty 4: Modern Warfare includes a class system which has five preset classes with default weapon load outs and perks assigned to them, as well as allows users to create five custom classes where the user can customize their weapon load out and three perks to assign to their soldier. The five default classes include Assault, Special Ops, Light Machine Gunner, Demolitions, and Sniper. Only two of the default classes are available to a first time player; the rest are unlocked as the player gains more experience online and gains higher rank. Create-a-class can be unlocked at level 4; players can name and save their own custom classes for quick access in multiplayer matches.

Experience Points (XP) are earned by either killing opponents or completing certain objectives, such as defusing the bomb in Sabatoge. The amount of XP gained varies depending on the type of match the user is playing. When the user gains enough Experience Points their rank/level increases. The highest attainable level is 55, which unlocks Prestige mode (which is only available for the Xbox 360 and PlayStation 3 versions of the game). In Prestige mode, the player can trade in their rank and start over at level 1, in exchange for a special in-game insignia by their name.The player can trade in their rank and start over at level 1 up to ten times, with the prestige insignia changing each time, effectively giving the player 605 total ranks to achieve.

Most of the new weapons and perks that players can use to outfit their custom class are unlocked as they gain higher ranks, but there are also a separate number of challenges associated with each weapon that subsequently open up new add-on gear as players complete them. These unlocks, such as camoflage or attachments for the weapon are unlocked after attaining a certain number of kills or headshots. Completing these challenges also gives the player XP.

Source: Wikipedia