4k not showing as an option

Find out from the manual if the HDMI 2.0 (or better) port in use supports 4K HDR HDCP 2.2. The HDMI ports on the TV may differ in functionality and capability. Possibly you need to configure the HDMI port on the TV for use with 4K high bandwidth use.

For the last Samsung TV that I checked, it was like this:

Home button on remote﹥Settings﹥Picture﹥Expert Settings﹥HDMI UHD Color﹥[select HDMI port]﹥On﹥Restart TV.

If you connect a 4K display, then Apple TV 4K should show 4K options in Settings.

If you connect a FHD display, then Apple TV 4K will show 1080p as highest resolution option.

If you connect 4K display, Apple TV HD will show 1080p as highest resolution option.

“High Speed” is a low end 10 Gbps labeling nowadays for HDMI cables. You’ll want “Premium High Speed” (18 Gbps) or “Ultra High Speed” (48 Gbps).

Verify your video settings on Apple TV: Use YCbCr with Chroma 4:2:0.

There is nothing to do, if the content is not being streamed in 4K then the Apple TV will only stream that content at the resolution advertised by the content provider.