I'm having the following problem which has been noted as a bug by Adobe support:
When creating a square image or artboard on a 4K monitor (mine's a Samsung U28-E590) the rendered image or artboard (and anything else really) is made longer in the X/horizontal dimension. So a 130 x 130mm block measures 130 x 135mm when measured with a physical ruler at 1:1 scale.

By simply moving the entire application window over to the Macbook side the proportions are displayed correctly and the X and Y sides are identical. This issue arrises regardless of starting up Ai on the Macbook Pro monitor side, or the 4K display side.

The issue persists with HDMI and DisplayPort, at MacOS's "Default for Display" setting and the "Scaled" settings. I'v ensured it's not a on-screen setting either that could be causing this problem.
With higher than 1080 displays like 4K and 5K becoming preferred screen sizes this should be looked into please. Unless I'm doing something wrong and support does not know how to resolve this, I see no other way than to report it as a bug.