How do I unlock 4G bands?
There has been an unlock method available for most Snapdragon 800/801 devices, and some forums claim it works for the MI 4 too. However, 1) it involves flashing a custom ROM and using a dangerous diagnostic tool, and 2) it doesn't unlock what the phone is not supposed to support (e.g. CDMA), nor is it guaranteed to unlock the band you want.

If you're still curious, take a look here.

And before you leave, you should know that MI 4 has 3G and 4G variants. If you have the 3G variant then just give up - there's no hardware.