DISCLAIMER - I've tested this on the MIUI 8.4.3 global beta (oreo) only, but i guess it might work on all miui versions. Please report your results.
Okay enough talking, lets get to it.
1. Open /system/etc/device_features/whyred.xml
2. Find the line that says "support_camera_4k_quality"
3. Change the "false" to "true", then save the file. (check attached screenshot)
4. Reboot and boom, you have UHD 4K option under video resolution
That's pretty much it! Cheers
Update: Here's a magisk module by @j1505243 if you're lazy