Hi I think the T8-S and plus only has HDMI 1.4, but you can still get 4k, but not true 4k as it's compressed heavily. The droidBOX Q8-S and new T8mini and MXQ pro have HDMI 2.0 which is the standard for 4k with 10 bit HDR and Dolby vision (found on Ultra blu ray).
Try playing with the Acceleration setting see if this helps, sometimes it's just the source that's the issue.
Good luck