Maximum screen resolution with AirPlay Extend Display on the Apple TV 4k? What is the maximum screen resolution possible with AirPlay Extend Display on the Apple TV 4K?

I.e. is it possible to treat an Apple TV 4K connected to a 4k TV as an external Retina-enabled display with a physical 3840×2160 resolution mimicking a 1920x1080 display, but with better font clarity etc?
I had some luck with my lg 65 inch 4k display.  By turning off underscanning in the Apple TVs Airplay menu I have gone from having bars around my 'external' display and this seems to have increased the clarity of text.

However, I am still faced with only scaled options for 1080p and 720 on my macs display options for Apple TV.

I guess there could be two reasons for this...

Requires a High Sierra Software update to enable 4K mirroring

The network bandwidth required for 4K streaming may be a challenge to ensure a smooth experience and hence why it is still not possible.

it would be nice to have the option either way....if it came with a disclaimer that performance may be delayed at least id have a choice to put up with it or not....