Exactly this. Screen tearing and also stuttering. It's kinda random though, really seems to depend on the content/video which is kinda odd. Like say a scene where the entire scene moves it might stutter a bit and have top and bottom screen tearing.
Like here's an example:
Start of video when it's panning it has lot of trouble with that. Even in HD. But since I play in full screen as far as the GPU is concerned it probably does not really know or care that the video itself is HD, it's still 4k content as far as it's concerned. Ex: it still needs to render it and send it to the monitor at 4k right. If I play it in a window then it seems to be ok as it's less for it to process I guess.
As a side note do cables make a difference or is that all marketing wank? I realized some cables are sold as HDMI 2.0 with 4k at 60fps support. Does it mean anything lower might not fully support 4k? Found conflicting info on that.