Go to http://www.evenbalance.com/index.php?page=pbsetup.php
Scroll down and find your Operating system.
You can either save the file onto your computer so you can update manually anytime, or you can just run it everytime.
Open pbsetup.exe and add the game you want to update. Then click check for updates.
After it updates you can close the program and open up Call of Duty 4 to see if the update worked.