PS4K will be a PS4 with 4K video. And if they want, devs (mostly indies) can make games in 4K for it, I guess. The Jaguar APU will stay the same.
In order to play today's 1080p PS4 games at 4K and the same framerate, you'd need a GPU roughly equivalent to a Fury X, which right now is top-tier, plus a better CPU (CPU and resolution load don't scale linearly, but they do scale).
That would likely mean a Polaris+Zen APU in order for it all to fit in a sub-250W TDP. A Polaris GPU with Fury X perf would likely be considered mid-high end, just like PS360 GPUs were at the time, which would mean you can bet your ass the console wouldn't cost $399. And neither Sony nor Microsoft are willing to bleed themselves selling hardware this time.
"But what if the PS4K cost Five Hundred Ninety Nine US Dollars? You can just buy the regular PS4 instead!"
The problem is that Sony's most likely making a new version of the PS4 in order to promote the 4K standard. Current PS4 hardware can already almost handle 4K content; all it needs is a HDMI controller refresh and probably some new encoding/anti-piracy chip, whatever.
Why would Sony make their standard-promoting product more expensive than it needs to be? It makes no sense unless Sony really wanted 4K gaming and 4K video was just a bonus, which is something I find hard to believe since they're comfortably leading this gen. You'll get your PS4 games in 4K - The PS5 is almost guaranteed to have backwards compatibility, after all, and patching in 4K would be pretty easy for most games.
