As Mr357 said, the game is being developed for the PS4, it has to be, otherwise we would have a hell of a lot of angry backers. The PS4 user's could still play the game, but it would be at a very big performance hit if it was dev'd with the higher specs in mind. If Yu wants to take advantage of the higher spec hardware of the "PS4K" (which IMO is a very misguided name) he will have to patch the game in able to do so.
Quote from the most recent rumor:
"It was also made very clear that current games would not be getting any type of performance upgrades by being played on the system and any benefits to older games would come via patch per game and per developer"
This would be a cool thing that he could do after the game has released just to up the res/fps (the engine already supports this I believe, as the PC version will take advance of higher specs).
If you are interested in the "PS4K" then I really recommend this digital foundry video as it clears up some reasons why there is noway that this new version of the PS4 will be ruining native games (good looking games) in 4k, and is most likely just a slight increase in GPU power to help with the PSVR or maybe to add some new technologies such a 4k video decoding.
Its a great video, very interesting, and easy to understand.