4A Games have created two huge demos, one each for for Metro 2033 Redux and Metro: Last Light Redux and they are available to download right now for PlayStation 4.
The team say the were inspired by the original release of Doom in which you got the first third of the game for free and then only paid if you wanted the remaining two thirds.
“We applied this idea to Metro – and so each trial contains nine whole chapters representing several hours of gameplay,” say 4A Games. “If you reach the end of the trial and want to continue, you can unlock the full version and play on without interruption – you keep your game save!”
Source: EU Blog