A representative confirmed the news to Joystiq at this week's PS4 event, telling the outlet that when Don't Starve hits the console in "early January" PS Plus subscribers will be able to download it and play without paying an additional cent. In addition, "a Vita native version with cross-save is coming a couple months after the PS4 version," the Sony rep confirmed.
The outlet also received confirmation that when The Binding of Isaac: Rebirth makes its way to PS4 early 2014, it will be free as well. I think it is safe to say the bulk of free games on PlayStation 4 will be indie downloadable titles for quite some time. At least until the library is a bit larger.