How to go back to stable rom from beta rom in Redmi Note 4?
I would suggest to apply for the unlock. The possibility of not being accepted is rather low. it takes around two days to get accepted but do not forget to go into the developer mode and pair the phone with the account because that needs to be done 3 days prior to attempting an unlock.

Now to the main question. You can download the zip for the version of the ROM you want to go back to and from here
Throw it in the downloaded_rom folder, go in the updater app, click dots on upper right corner, choose update package.

Now... there is a slight possibility that it might not work, failing when it tries to verify the package. If that happens then unlocking the bootloader is your only option.