Apparently, Sony has stopped development of the fourth game in the main Insomniac series for PlayStaiton3-Resistance. The company reportedly did this because it was concerned that it had too many post-apocalyptic games at the time.
This was covered by Colin Moriarty in episode 153 of the Sacred Symbols podcast. Moriarty notes that the abundance of post-apocalyptic games from internal studios (Killzone Shadow Fall, Horizon Zero Dawn, Days Gone, and The Last of Us) that were in development or were in preparation for production was a problem for Sony.
“Resistance 4 was presented at the time by Sony, but they rejected development because it sounded too similar to The Last of Us, and they didn’t want the two games to overlap. Resistance was canceled at this point. “