Private Matches of SAS: Zombie Assault 4 were added in the August 7th, 2014 update. They, literally, allow private matches so players can play with who they want to as long as they have the password/game ID. The maker of the game chooses the map by clicking the pictures that represent each mission. A checkmark by the top-right corner shows you have chosen that map. Once the map is chosen, you can still change the map. The map starts building when the match-maker clicks the "Start Game" button. Like normal games, 4 players can join a match, after the building stage is done, the game is exactly the same as a normal multiplayer game.
When creating a game, the name is made for you, and is composed of 8 digits of random numbers. When joining a private match, if the game name does not exist, a game with that name is created. Whoever joins first becomes the "host" and decides the map and when to start. If the player is banned, creating a match will always give out code 101012 and they cannot join any multiplayer matches.