The D850's FX BSI CMOS sensor allows 4K ... fixed to DX format (APS-C crop), regardless of lens used or image area selection. Nikon is touting both 4K and 8K timelapse, and it's the latter that ...
The D850 still relies on contrast-detect AF in Live View mode, though, so AF speed will vary a lot depending on the lens being used, and that is why we typically don't measure Live View full AF ...