The rules for sharing games on the PS4 platform are sounding a bit complicated. Although, sharing the physical game is still just as simple as lending the disc to a friend. Sharing downloaded content is going to be a little more confusing at best.
Sony says that its users will be able to download content to as many consoles as they so desire, but only two instances of that game will be able to be played at the same time, one game being played on the primary console and the second copy of the game on a secondary console. One caveat to this is that the primary user must be logged into both consoles for the games to be played concurrently.
Being a person that has download a few games just to play by myself, I really do not see much issue in what Sony is proposing. I generally only download single player games, but I can see where this might come into play a bit more when multiplayer comes into play.
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