Journey's coming to the PS4 later this year, along with cult PS3 fave The Unfinished Swan, according to a post on the official PlayStation blog. A line in the YouTube video's description mentions cross-buy: "Get more for your money with Cross-Buy - purchase this game for PlayStation 3 or PlayStation 4 and get the other version at no extra cost." We've reached out to Sony to confirm.
Update: A Sony rep has confirmed that Journey will be a cross-buy title, saying "Journey is a cross-buy digital title. If you have downloaded it digitally on PS3, you will be able to download it for free on the PS4, much like how fl0w and Flower were launched for PS4."