Here are the keys for playing multiplayer on Call of Duty 4. Here's the catch:

• You can't play over legit servers. Only cracked or hacked servers. (You can play this over Garena and/or Xfire or any other cracked server.
• There may be prominence of Hackers and will likely ruin your gameplay. Most open servers don't have punkbuster enabled. 
• Download at your own risk (although it is safe, I cannot guarantee if the link will be up again as my last post on 4shared got lots of complaints)

Download Here