Thank you for your comments, but I have found the answer to my question by doing some more digging into UHD TV specifications.

If I set the video output of my box to 2106p, this is a valid 4K UHD TV standard and will ensure my TV displays the 3840x2160 content correctly.

Out of interest, 2160p resolution is in reference to the 4K UHD TV resolution of 3840×2160, or a vertical resolution of 2160p for an overall image aspect ratio of 16∶9.

True 4K resolution of 4096(4K) x 2160 gives an aspect ratio of 256∶135 as defined by the DCI Digital Cinema System Specification for use in cinemas.