This is the method i use , Start the game with 3 people in it make sure matchmaking is closed , as soon as you get into the match invite a 4th player , when he joins he will be spectating , when he joins blow up the files with sticky bomb or grenade laucher any explosive , the mission will end and you will recive the money etc , (this is important) all the players who were playing should press the like button and then continue , the spectator should press replay when the option of all the other missions comes up , rinse and repeat