Just a moment...
Generally, a multi-purpose file playing program should be able to handle playing back the footage; for .MOV files you may want to use Apple's Quicktime Player; for MP4 files you may want to use something like VLC Player.
Playing 4K or UHD footage can be extremely processor-intensive, and may benefit greatly from a modern graphics card. Which means — it's possible that your computer just may not be powerful enough to properly play 4K or UHD footage. Furthermore, unless you have a very modern monitor that's capable of 4K (or UHD) resolution, you may not be able to see the full detail and quality of the footage on your computer. There's no solution for that other than to try to play the footage back on a system that's fast enough to properly display it, and on a monitor that's large-enough and high-enough resolution to properly display the footage. On smaller or lower-resolution monitors, your video playback software can be configured to "full-screen" mode, in which it will automatically re-size (or "scale") the footage to fit the resolution of your screen. That will enable you to watch the footage, but not at 100% quality.