Check to see if your viewer in FCPX is set to "Better Performance" which displays a lower resolution when in motion for playback. If it is, change the setting to "Better Quality". I assume you are not using proxy (1/4 resolution) files, but that can be changed here also.
Agreed. Sounds like the system is dropping the screen resolution to play the 4K back smoothly for you. A simple test is to render a section of your shot as an H.264 MP4 and play it back then. All being well it will play back as sharp as it looks when you pause the playback head in FCPX. Best of luck.