This achievement REQUIRES 4 real players.
The four possible masks you can earn from the Hotline Miami "DLC" are:
Aubrey, Dennis, Graham, Rasmus.http://gyazo.com/4bc729f4d05c1488e7706b87eb3b8da0
(Ignore the text under the masks. That is PC only. To get these masks, you must get them from card drops at the end of a heist.)
Essentially, keep playing until each of you and your 3 friends gets one of the masks, then play the Hotline Miami heists on hard or above difficulty and it will unlock at the end. (YOU CANNOT WEAR DUPLICATE MASKS. EACH PLAYER MUST HAVE A DIFFERENT OF THE 4.)