Xiaomi hasn't released any Mi4 Windows 10 phones so far. Actually, Xiaomi forum has unveiled a custom ROM based on Windows 10 Mobile operating system, that can be installed on any LTE-version of Mi 4 (4G). Unfortunately, you can't install this software on Mi 4W (HSDPA-only model), especially from India.
If you want Windows 10 OS on your Mi4, you need to buy a LTE (4G) version of Mi4 and flash the software on it. Here is a step by step guide to install Windows 10 on Mi4: http://en.miui.com/thread-189574-1-1.html