The A.I. is something which has to be touted heavily as the A.I. in Call of Duty 4 is some of the best seen in a FPS to date.  The enemies are not only larger in numbers but they are also extremely intelligent.  In more than a few instances I saw the enemies move their positions based on what I was doing and put themselves in different spots, superior spots, each time to get a better line of sight to take me out.  They are extremely resourceful in that they will know how and when to use their surroundings.  On the higher difficulty levels, it is very difficult to take some of these guys down.

The way in which the multi-player of Call of Duty 4 is set up is based on classes, but with a twist.  The classes are not split up into your normal, medic, heavy gunner, light gunner classes.  Instead they allow for a lot more customization.  You can start out with a primary weapon, special grenade, and them from there you get special skills to go along with your particular class.  The classes do not prohibit you to do anything though in the actual games.  A lot of the choices you make will depend solely on which guns you are most comfortable using. 

The multi-player is the most enjoyable aspect of Call of Duty 4 and to be quite honest is absolutely addicting as hell.  There are sixteen maps in the game which you will enjoy playing through.  Each are extremely unique and will take practice to learn them.

The multi-player allows you to expand on your classes as you move up ranks the more kills and points you earn throughout your play online.  Once you hit level 4 you will be able to create your very own class and assign weapons, skills and such to your liking.  There are also challenges in place for each weapon which also opens up new things. 

The challenge system in Call of Duty 4 online is extremely enjoyable as it allows you to do more than just try to jump online, kill everyone, and jump off.  You will be trying to pull off various things such as killing people with a certain number of headshots in a row, using certain weapons, killing from various locations, etc.  You will keep playing and get into the fact that you can get achievements of sorts in-game to keep gamers filling up online games for a long time.  The same goes for weapons as the more you use a weapon the more it will be upgraded.

Multi-player also features some in-game challenges which give awards.  If you complete three kills in a row you will be able to get some UAV going for your team, allowing you to get radar of all of the locations of your enemies.  Get five kills in  row and you can call in an air-strike.  You get the idea here, this promotes good solid strategic gaming online as gamers can earn kills and really make it a team effort.