I've got a series 9, and although it superficially seemed to play 4k I was getting artefacts.
Check Settings for video on Apple TV and try manually setting 4k with 24 fps and work up as mine did not like 60 fps - I'm not discounting an inadequate HDMI lead.
The 24 fps setting is at the bottom under 'other' or similar menu at the bottom of the main list or at least was.