[JA]

The D850 would definetly be the "best", especially with the resolution advantage it has over the 5DS MkIV. In terms of noise they are both similar but if you view the scene comparison on dpreview you will see the D850 clearly sharper, but not necessarily "quieter". If you want full frame and 4K you should also consider the Sony A7R III. Attached is a scene comparison from dpreview of some fine text. Unfortunately the JPEG down load and upload here has blunted some of the extra contrast and sharpness in the D850, but you can still see it's higher resolution and higher limit before moire. USe the studio scene comparator on dpreview to view with less image degradation.

To be fair in the comparison below the Sony A7R III is in Normal mode. If it were in Pixel Shift Mode (4 images stacked & processed out of camera) it would be number 1 in the comparison.

Best
JA