The original version of the Mi 4 used the MIUI v5 operating system, but over time it was also upgraded to MIUI v6 and MIUI v7 versions. So, if you buy the Mi 4, it probably uses the MIUI v5 operating system. However, you can also get subsequent versions of the MIUI operating system through software updates. Therefore, it can be said that the Mi 4 May initially use the MIUI v5 operating system, but it is not necessarily fixed. Users can update to the latest MIUI version according to their needs and preferences.