Back on the WinHEC in March of 2015, Microsoft announced it will offer Window 10 Mobile Rom for Xiaomi Mi4.
After waiting for months and testing, the Beta version of Win 10 Mobile for Xiaomi Mi4 is released today. Last night, President of Xiaomi, Lin Bin, claimed on Weibo (Chinese Twitter) that the Win 10 Mobile for Xiaomi Mi4 will be released on December 3, 2016.
It is just unsure that Xiaomi Mi4 can get back to Android OS after flashing a Win 10 Mobile OS. It is a problem to be confirmed, or to be researched by the geeks of XiaoMi Fans.
You may find something about it on Xiaomi MIUI website.