I decided to start playing Worms 4 again but I can't find the slip of paper with my disck key on it. Is there any other way to play without it?

If the game is already installed on your computer you shouldn't need the CD key.

Well the thing is I uninstalled it a long time ago so i had to reinstall, so i still need the key.

I suggest contacting Starforce, and sending them the box+cd, and see if they can generate a new code for it.

I suggest contacting Starforce, and sending them the box+cd, and see if they can generate a new code for it.
Probably be better contacting Codemasters, the publishers (assuming you bought it in the UK): custserv@codemasters.com I believe.