4K video file exported from Final Cut Pro cannot be located in iTunes
It will be in the "Home Videos" section of iTunes with the same name as the Project Name from iTunes.

Worth checking with FCPX where it believes it exported it:


Select your Project, and in the Inspector window click the "sharing" button (3-headed arrow, top row of the inspector, next to the "i" button). 


If FCPX published it to the iTunes library, it will be confirmed there. If it published it somewhere else, it will show the location.