Jonathon Blow has shared some interesting new on upcoming 'The Witness' patch. Basically, PS4 Pro support is in the works, but it's also possible that HDR support for all PS4s could be happen as well.
In terms of PS4 Pro support, 'The Witness' will distinguish between a 4K connected PS4 Pro and a 1080p connected PS4 Pro. The specifics have not been finalized, but there is an outline of what the aim is for both situations.
here’s what we are aiming for:
When you start up the game on a PS4 Pro, it will go into one of two modes, depending on the current display device:
* When using PS4 Pro with a 4k display, we will render at 1440p or possibly higher, all while keeping a consistent 60 frames per second. This image will then be upscaled to 4k, and we will draw text, menus, and other UI at native 4k resolution. (This is similar to what happens on the base PS4, where we render the 3D scene at 900p, then upscale to 1080p and draw UI at 1080p). We might be able to achieve resolutions higher than 1440p based on various engineering tradeoffs (for example, if we boost the resolution of the main scene to a greater degree than we boost the resolution of the reflections in water, which tend to be distorted anyway. Also, we’ll evaluate checkerboard rendering — we’re not sure if it’s the right way to go for this game, but we’ll see.) We’ll post again when we finalize these specs.
* When using PS4 Pro with a display that is 1080p or lower, we will render the scene at 1080p (instead of the old 900p), and increase antialiasing quality from 2x to 4x MSAA, again while keeping a solid 60 frames per second. We are exploring additional visual quality increases, like increased streaming distance for high-resolution meshes, or decreasing the noticeability of the fade-in between faraway LODs and high-res close-up meshes. Again, we’ll post details when this is finalized!
We’re also working on HDR support. Here in the office we just got a new LG B6P television, which is very bright and has low black levels, providing a lot of dynamic range, and it’s been fun so far tweaking the look of the game on HDR displays like this.
So here again, a developer is taking some care to offer a nice performance bump for an already released title, and that's true for both 4K HDR sets and HD sets.