YT defaults to lowest quality unless you change it in settings, have you looked at the quality options, also it may only play 4K on TV. I've just checked and plays in 4K on the Youtube app in NowTV.
I have one but don’t use it, I use windjammers (dead cat) as the 9 has two microphone that need covering. I think they work really well and you get a lot more for your money.