***Same issue but side note to solve input issue from streaming and casting devices***
If you are using a stick to stream with of some sort I just spent some time trying to fix my lines and I went into the stick settings and it said "always play as HDR"
I turned that off and now streams are playing like they are meant to as some cartoons are made in 720 or 1080 and by forcing 4K it was trying to upscale something that wasn't there resulting in flaws in the picture and lines.
It will still play 4k streams if they were shot or encoded in 4k or made in 4K depends how you set it.
I've just turned mine off or you could also set it to 1080 at 60 frames per second or auto and I've gotten rid of the issue and everything looks incredible again.
Cartoons are poppy and beautiful without any artifacts or lines or noise and live action is much better too as it's playing whatever format it was exported to for streaming if that makes sense.
I have a Hisense 4K 75 inch screen. It's now flawless I must have changed the setting in the Roku, Fire stick, Chromecast (whatever device you use) by accident thinking it would make the picture better. I was wrong you need the video files to play in their native formats just like Nolan intended.
Hope this helps someone