"PS4 Link" is the name given to the PlayStation App for PS Vita by Sony's Shuhei Yoshida. It's a PS4 download manager and communications tool, that is also coming to iOS and Android. Yoshida has stated that it's coming to Sony's portable in the next system update.
When asked about the app's launch on PS Vita by a fan on Twitter, Yoshida replied:
@OsAndRavensFan1 We'll add "PS4 Link" app to PS Vita with the next update.
— Shuhei Yoshida (@yosp) October 5, 2013
It remains to be seen if "PS4 Link" is merely a working title or the actual name for Sony's app.
You can see shots of the app's interface and get details on what it does here.