Not saying your answer is wrong, per say, but I would like to add something to this topic because I've had the EXACT same issue. It's maddening. But, I believe I've figured it out, and your answer is mostly correct. What I've found is that playing 10bit 4k hdr content files through a video program (like plex or vlc) will allow you to play the 4k content downconverted to your native resolution (mine is 1080)...and the issue is resolved. So, at least for my display, I know it's a 10bit display that is having massive issues with color gradients while displaying anything below 10bit. 8bit content displays with absolutely horrendous color gradients, unless you get a pure source of the file you're trying to play and mess around with handbrake and convert from that. I managed to minimize some 8bit issues with color gradients through using handbrake on the third season of Black Sails. Most files have corners cut on them when initial conversion happens (to shorten the file size), but this aspect alone also introduces terrible artifacts and color gradients. The gradients are somewhat noticeable still, but this is also a fix for the content (re-convert the file in handbrake at MAX settings for video fidelity: will take maybe 4 to 8 hours per hour long episode).