Playstation.com's description for Batman: Arkham Knight "Terror Unleashes" strongly hints the Scarecrow DLC will be available for PS4.
As a matter of fact, that is exactly what it says:
Gotham falls victim to Scarecrow's fear toxin, transforming the famous city into a twisted, hellish nightmare in the Batman: Arkham Knight Scarecrow Nightmare Pack.
Tear through the city in the all-new batmobile and face off against a towering vision of Scarecrow and his undead army as they spread terror on the streets, available exclusively on Playstation 4.
Batman: Arkham Knight is based on DC Comics’ core Batman license and will be available in 2015 exclusively for the PlayStation®4 computer entertainment system, Xbox One, the all-in-one games and entertainment system from Microsoft, and Windows PC.
In the explosive finale to the Arkham series, Batman faces the ultimate threat against the city he is sworn to protect. The Scarecrow returns to unite an impressive roster of super villains, including Penguin, Two-Face and Harley Quinn, to destroy The Dark Knight forever. Batman: Arkham Knight introduces Rocksteady's uniquely designed version of the Batmobile, which is drivable for the first time in the franchise. The addition of this legendary vehicle, combined with the acclaimed gameplay of the Batman Arkham series, offers gamers the ultimate and complete Batman experience as they tear through the streets and soar across the skyline of the entirety of Gotham City.