The rumor stems from a tweet by Shinobi602 that reads:
Well…Horizon: Zero Dawn’s getting delayed to early 2017 from what I’m told. Going to look splendid on PS4k though.
The NeoGaf response indicates that the game isn’t being geared towards 4K or the new PS4, it will just look great and be a good example of the new hardware. The old hardware will look fine as GG always does a quality job testing games performance.
That being said there isn’t much else to go on regarding the delay except maybe GG’s track record. Killzone was often delayed before release and had many tweaks before a final release was made. We also won’t hear much about the delay right now since convention season is picking up, and E3 is right around the corner.
Overall I’m a bit disapointed if this one comes out to be true because it was a game I was highly excited for.