I don't think I've ever heard of someone trying to do that, 4 players on the same mobile, not taking it in turns but playing at the same time? Sounds crowded. if i were to do it i'd split the screen into 4 and have 4 invisible sprites over each section, when you touch anywhere on sprite A, player A can move to any x,y within that area, touching sprite B only moves player B to any x,y within sprite B etc