The update will be 100MB in size, so the customers are recommended to download it over a Wi-Fi connection. The build number will change to MMB29M post the update, while MIUI 7.1 will remain the stock UI.
Xiaomi recently sent out the MIUI 7.1 update to its devices, which indicates that the users of the Mi 4 might not see any major changes on board. But the default Android 6.0 related system changes will be apparent after the update is installed.
The Mi 4 has certainly come a long way since its release when it was running Android 4.4.3 KitKat. It was updated to Android 5.1 Lollipop about one year later (October 2015) and has now finally seen Marshmallow.
Xiaomi is also expected to rollout the update to other devices like the Redmi 2, Mi 4i and the likes. The update might be rolling out in stages, with only Chinese models seeing it as of now. In any case, the rollout should complete by the next few weeks.