How to play Left 4 Dead Co-op mode with Hamachi

One person needs to create a hamachi server and allow the other 3 players to join it before you continue to the next steps.

For the Host (1 person)

1. Go to the folder that you have installed Left 4 Dead.
2. Create a new text file in this folder

left4dead.exe -game left4dead -console -novid +sv_allow_lobby_connect_only 0 +z_difficulty Impossible +map l4d_hospital01_apartment

Save this file as Launch.bat (it is very important that you save it as a ".bat" file). This will be the file that launches your game.

Run Launch.bat

Go to the next time and tell your friends to join you.

For the Clients (1-3 people)

1. Launch Left 4 Dead
2. Press the button "`" (top left corner of your keyboard) to get into the Console
   
3. Type in connect HAMACHI-IP-ADDRESS (For example: connect 5.111.216.175)
4. You will now be connecting to the host

Enjoy!