I think that the 5D4 will beat the 6D in dynamic range, and slightly edge the 6D in low light /noise performance. IMO the main advantage between the two, besides the 5D4 having higher MP, would be the AF points (61 vs 11) and cross-point AF (11 vs 1), and dual cards.
The R6 mark II is slightly larger and heavier than the A7 mark IV. Both cameras offer weather sealing and are built with a magnesium alloy chassis. R6 II: 138.4 Γ 98.4 Γ 88.4mm, 670g. A7 IV: 131.3 x 96.4 x 79.8mm, 658g. The Canon design is my favourite mainly because of the larger, rounder and taller grip.
Here's the Nikon D750 @ 25,600 vs the 5d mark iv @ 25,600, both with 50 luminance NR in lightroom. Very similar. Note: for some reason the NR looks worse in the exported jpeg from the 5d mark iv than what I see in lightroom before export. Not sure why. When viewing in lightroom there's less of a difference.
Landscape (Dynamic Range) i. 11.9. 9.6 Panasonic Lumix DMC FX150. BEST 14.8. Top score Hasselblad X1D-50c. Canon EOS 5D Mark II vs Canon EOS 1Ds Mark III
. 877 717 260 920 664 349 827 405
canon 6d mark ii vs 5d mark iv dynamic range