Netflix announced their capability sometime ago and even though I bought a 4K TV a few months ago, it was only a few weeks ago that I tried out the capability.
I have to admit I spent some period of time trying to figure it out. I could not get the 4K option work on my XBox One or FireTV, etc, and then finally read up and realized I had to use the Netflix app in the Samsung TV itself to see the content options.
Netflix looks gorgeous in 4K. There is a dedicated "row" of content options for 4K. However, despite the fact that I have 45 Mbps of bandwidth, it does buffer and crash the app...a lot.
Amazon, which announced their option only 2 days ago, is a much bigger challenge for content discovery. I cannot find a 4K area on the UI, but was able to search for content on my phone. Ironically, looking up a CNET article was the best source for finding available content. There was a lot of rebuffering, though unlike Amazon, it did not crash the app.
I did a bunch of comparisons to 2K content on the Xbox and was hoping the 4K upscale would be nearly as good, but it wasn't.
So I am glad the shift to higher quality content has started, but we are a long ways from this being a viable, main stream content option.