Question Serious battery drain

[quickitie]

If your device dies in less than a day, return or exchange it. It's a hardware defect.

 

[tomini]

Now my band drop 20% per hour, i will try to discharge and charge for a few cycle before contract the seller.

If you use modded MiFit, try Factory Reset the Mi Band 4 (in settings on Mi Band 4 display), disconnect band in MiFit after that and then connect again to MiFit and let it update.
It helped me.
If you use Tools & Mi Band app, then you need to copy Bluetooth address from MiFit, it's in the Mi Band 4 settings down above DISCONNECT button and copy it to the Tools & Mi Band app in settings.