Will be interesting to see how this plays out. They seem very cocky about this, making sure they get a dig at Sony, yet they are still a year away from Scorpio.
4k is extremely resources heavy, even pushing something like a i7 6700k and GTX 1080 or Titan X to its limits (no 4k/60fps Ultra yet) If they are forcing devs to do native 4k then they might get games that don't look any better than the PS4 Pro (Graphically) and depending on Scorpio's price, will give Sony the advantage again, as Pro will probably be cheaper too.