I installed "open camera", I think it is from the Play store on a Samsung J5 and there is a setting:
"Force 4K UHD video (only works on some devices)"
"Enable 3840X2160 resolution for video recording on back camera - this option is a hack that may allow 4k devices on 4k cameras that don't expose the option to 3rd party camera apps. This isn't guaranteed to work, please test before working."
When I increased the resolution and decreased the DPI after unlocking and rooting it some of the bundled stuff didn't work or display properly so I found that some better stuff on the play store and open camera was one of them.
What's nice as well is that free "Samba filesharing" (requires unrooting) where I can copy and move files directly over when it is near an access point after taking videos and pictures without using some "freemium app" to do it for me just like with the Toshiba Flashair sdcards.