It seems the subtitles problem in 4K is fixed in 11.1 update ... If not I recommended I totally reset ... It fix my problem 
A word of caution for us Scandinavians using Viaplay/Viafree: I just updated to tvOS 11.2 beta, and it breaks video playback in at least Viafree. When you try to start a video, it just takes you back to the Apple TV home screen. I wouldn’t count on the app being updated quickly, so my recommendation is to hold off on 11.2 until full release in about a month or so.
The best thing you could do is pressure apple and netflix about it.
I had the problem and talked to netflix for aprox 10 hours before they found out it was a software problem. It now works. Im on 11.1
Well the 11.1 update did solve the problem for me