As many of you may have noticed, steam recently had an update for CoD4. The update was intended to fix a pretty bad expoit with overflow limits but instead caused more issues than it solved.
To fix this issue
Download the new CoD4 exe file from Google Drive or from the attachments bellow
Open a new Windows Explorer Window. Go to your "Steam" folder (Which is likely in "C:\Program Files(X86)") then click on "steamapps" then "common" and then "Call of Duty 4". Here you got to your CoD4 folder.
Then simply drag and drop the new .exe into the games directory and let it replace it.
For more info on this issue you can read go here
iw3mp.exe