You can create price alert here, copy coupons and view price history. What is the best price?
Charging Time: About 2hours, Resolution: 240*120pixels, Standby Time: About 20days, Bluetooth Version BT 5.0, Using time: About 7days, Size: 0.95 Inch, Type: AMOLED, Battery Capacity: 135mAh
Aliexpress
There’re some similar (or not) items on Aliexpress
Click here to search by yourself. Here are coupons that you can use.
Latest price update (UTC time):
Banggood
Banggood warehouse is located in Hong Kong. I found coupon for you. Check this out.
Link: Banggood product page
1. ($29.69): band10
you should apply the coupon in your cart after clicking “Proceed to checkout”. Click to copy code and open shop.
Exclusive Allowance Coupons: https://bit.ly/3xWgD9k and https://couponsfromchina.com/short/5cd
Old coupons (from the oldest to the newest): BGeu123, BGmib4Andre, TTxBG10, BGmiband4cina, BGYAmband, BGmiband4grsd, BGBANDPL1, BGktechmib4, BGlskdeimi, BGM43C, bgcn4, BGletni4cn, BGplmiband4, BG3CSUM, BGM4d1, BGNApb05, BGM4FX, BGMICN4, BGDQMI4CN, AFfbG10, BGM428, BGM427, 502C6B, BGBAND29, BGPL11BAND, BGBFAFF2019, BGPro19, BGXMI4, BGNYMI426, BGVDay10%off, BGM426, 88c16e, BGMI4NOVOPRO, BGBGM426CN, BGAFF10OFF, BGMB4M, 9dbbc6, BGM4JL, BGM424, BGEEMIBAND4, BGM42399, bgshOOp10
(there’s a little possibility that old coupon will work)
Price history and price alert
Here is price history (for all/most countries) for Xiaomi Mi Band 4 CN Smart Bracelet.
Price records source(s): Gearbest, Banggood
Click on image to enlarge. 
You can set price alert (for Banggood and Aliexpress (only if product name will be the same as on Banggood and I will post it on social media though, so it’s more like Banggood price alert)).