It’s not as simple as owning a 4K TV and plugging a computer in. This can be especially frustrating when you just want to relax and watch UHD content via your high end GPU instead of the tiny GPU in your smart TV.
I found a handy article on HowToGeek that covers just about everything. In addition to the steps listed there, you’ll need to head on over to the Microsoft Store and get the HEVC Video Extensions from Device Manufacturer. https://www.microsoft.com/en-us/p/hevc-video-extensions-from-device-manufacturer/9n4wgh0z6vhq?activetab=pivot:overviewtab
I also recommend checking this page if you’re an Nvidia user: https://nvidia.custhelp.com/app/answers/detail/a_id/4583/~/4k-uhd-netflix-content-on-nvidia-gpus
AMD/ATI users should check requirements here: https://www.guru3d.com/news-story/amd-adds-netflix-4k-support-to-rx-5700-video-cards.html