I have a 2019 Vizio V505-G- 4K HDR TV stand-alone Roku Ultra Model 4640 X Software 9.2.0 Build 4807-29 (as of 2/9/2020) hooked up via 8K HDCP 2.2 certified HDMI cable. I am capable of watching 4K, but the output is spotty at best. Netflix does not stream at all in 4K, YouTube has no problems. I am currently watching Better Call Saul on Netflix on my Smartcast App on the TV and the tags at the top register Ultra HD 4K. On the Roku Ultra, it does not and streams only on 1060 HD. Strange? But guess what, why can my TV stream in 4K HDR w/Dolby Vision and the stand-alone Roku Ultra can't? I was reserved for not using the built-in Netflix app on any TV because of poor performance in the past. Now it's just the opposite. My TV HDMI (HDCP 2.2) connections in the TV are enabled for HDR. Audio set to Auto. What's going on? I paid for 4K performance and I am not getting it!
The problem isn't Roku Ultra. It sounds like you need to upgrade your Netflix subscription so that it will stream some (not all) content in HDR10/Dolby Vision formats (ie: 4K). Once you do that, you'll see Netflix content such as the Marvel shows like Luke Cage is indeed available in Ultra HD or Dolby Vision. But you have to pay to play as it'll cost $15.99 to upgrade to the 4K streaming through Netflix. But it's all worth it.
Actually, Roku Ultra doesn't support Dolby Vision yet. Which is strange since Roku is the top streaming box on the planet. I think there's a licensing issue with Dolby that's causing the problem. So Roku Ultra can ONLY support HDR10 (ie: 4k). Nevertheless, you still need to upgrade your Netflix subscription to the 4K streaming service which will cost you $15.99 a month. Once you do that, you'll see the 4k content on Netflix.
I have been watching several series and movies in 4K HDR Dolby Vision on Prime Video using a 4K Fire Stick. However my Roku Streaming Stick+ doesn't apparently support Dolby Vision...The difference in color gamut between Dolby Vision and "Standard" is huge.
When will Roku support Dolby Vision (or should I go back to using my 4K Fire Stick to watch 4K content)?
I suspect Roku will have the licensing with Dolby straightened out when they reveal the next gen Roku Ultra devices this fall. Until then, HDR10 is the way go to. Unless you get an AppleTV device. Their boxes support Dolby Vision too.
HDR10 and Dolby Vision are two different formats of 4K. HDR10 is open source while Dolby Vision is Proprietary. Meaning, companies like Roku and Amazon has to pay a licensing fee to Dolby to use Dolby Vision on their devices. While HDR10 is free from licensing.
So, until Roku can work out the licensing fees with Dolby to use Dolby Vision on their devices, Amazon 4K Fire stick is the way to go.