I have the same situation. I have a Samsung 4K HDR10 compatible TV (Samsung does not support the Dolby Vision 'standard') connected to the Apple TV 4K via HDMI.
When streaming Netflix content, over 100 Mbps Comcast internet service, I have no problem streaming shows like Breaking Bad (non-native Netflix 4K content) in 4K. The same holds true for all Netflix content that shows me an Ultravision 4K logo on the title screen for the film. If the content shows to be 4K on the title screen I see 4K quality on my TV.
If the content shows the Dolby Vision logo on the title screen and not the 4K logo, the stream looks like it is HD, or some resolution that is less sharp and brilliant than the resolution that I see when viewing content that shows a 4K logo on the title screen.
So, what does the Apple TV 4K device do with Dolby Vision format when sending it to a device like my Samsung which only supports the HDR10 standard? Also, is there a way to verify what format the Apple TV device is sending to my TV?
There is a bit more to this situation with regard to HDR10 & Dolby Vision Netflix streaming. Original Netflix content such as Stranger Things, Mindhunter, and all of the Marvel shows have, in the recent past (prior to the release of Stranger Things season 2), displayed the Utravision 4K logo on their title screens on my current setup. They also were streaming in eye-popping 4K. But now, all of the original Netflix offerings only show as Dolby Vision, and are in 4K resolution when they reach my TV. I had a long discussion with Netflix support about this and they could not explain what was happening.