To stop the box reverting to 1080p go Settings - More - Display and untick 'optimal display format'. This is the toggle for auto resolution.
Kodi will always say 1080p because it doesn't support 4K yet. However when you play files through the wrapper they will play at 4K.
With the file playing through the wrapper (or using the media player app from the box main menu) where are you seeing that the 4K file is 1080p? If it's when you pause the video at the top left (e.g. 'Video: 1920x1080') then this is the file resolution, the app won't misreport the resolution. What resolution does the TV say it's operating at?
Download the 4K Sintel sample from here http://www.libde265.org/downloads-videos/
What does the wrapper / media player app report as resolution?