You should set it to what works for you. That is why it is configurable.
For 4K support, set Apple TV Format to 4K (3840×2160).
For the refresh rate (Hz), set Apple TV to 60 Hz in NTSC countries, or to 50 Hz in PAL countries.
If your TV does HDR or Dolby Vision, then I would suggest to set Apple TV to SDR and turn on Match Dynamic Range.
If your TV supports the various offered content frame rates, then I would suggest to set Apple TV to Match Frame Rate.
I would suggest to set Apple TV Color Space to YCbCr (not RGB), as the source video will be YCbCr, and your TV can handle it just fine.
I would suggest to set Apple TV Chroma to 4:2:0, as all streaming video is 4:2:0 in their file, and other chroma settings would not increase video quality, but only increase bandwidth use (possibly at the cost of other video properties).
About frame rate and dynamic range matching on Apple TV - Apple Support
About 4K, HDR, and Dolby Vision on your Apple TV 4K - Apple Support
Adjust video settings - Apple TV user guide - Apple Support