Update: Sony has confirmed that Journey will indeed arrive on the PS Store on July 21.
Journey will support cross buy. If you have digitally purchased it already for PS3, you can download it free for PS4.
What the new trailer below.
Original: The PS4 version of Journey is set for a July 21 release, according to Docs for PlayStation.
Sony's official support documentation shows the date quite clearly, although whether it has been changed since these documents where published remains unclear.
The PS4 version of Journey includes 1080p graphics and a 60fps frame rate, and will be available as a Cross-Buy title if you previously purchased the PS3 version.