It seems that Sony has been openly inviting free-to-play games to make their way onto the PS4, and it looks like one of the premier F2P games has found new success on Sony's next gen system.
According to Associate Producer Pat Kudirka Sony’s new platform is another successful venture for the game, as he mentioned in a comment on the PlayStation Blog.
"To answer your question we’re having a lot of success on both the PC and the PS4. The PS4 sign-ups continue to grow daily and I think 2014 is going to be a big year for free-to-play titles on the PlayStation 4. Thanks for playing Warframe, we hope you enjoy this latest update!"