The addition of perks brought a new perspective to gameplay as well. Also once you reach ceratin kill streaks (which are kills you gain without being killed yourself) you gain the UAV or Unmanned Aerial Vehicle which can spot enemy units on the map unless a soldier is using the 'UAV Jammer' Perk, Air strikes which calls in 3 waves of jet fighters to drop bombs on a target and finally a helicopter which roams around the map shooting enemy targets.
Unlike in previous versions, you can gain ranks (all the way up to commander), which provides you with new weapons and other unlocks for your weapons. I also found the single player storyline to be very dramatic, with some great new elements added that I had never experienced in a game like this before (main characters dieing for one). The single player puts you in many different situations, starting from training for a raid on a cargo ship looking for a nuclear device, hostage rescue and assasination. This version sees you playing as a member of the SAS and Marine Force Recon.
Call of Duty 4 is the latest in the Call of Duty Series and was released in November 2007.