You are right that per-pixel noise mainly depends on pixel size (most digital cameras are shot noise limited at high ISO). However, we should be comparing imag

You are right that per-pixel noise mainly depends on pixel size (most digital cameras are shot noise limited at high ISO). However, we should be comparing images at the same size, rather than at 100% pixel crops. If you print the high resolution 5Ds images in the same size as the 5D4's, the noise will be finer and less distracting. You can also denoise and downsample the image to the same number of megapixels as the 5D4's and get a similar amount of noise per pixel. In this sense, a high resolution sensor with smaller pixels is always better because you get the freedom to have finer detail in bright light and still get clean images at lower resolution in dim light.