Miui 9 works very well. It feels very good a speed boost. It really does not feel the need for Android 7. Being based on Android 6 Xposed modules works OK.
4x note has the same hardware as red 4 pro / prime. Note 4x has Android 7 but Redmi4 prime, no. Can you put the rom of Notes 4 x on Redmi4 prime?
Miui 9 works very well, but how to integrate Android 7 was even better. I think it's possible.