Aliexpress
Here are coupons that you can use.
Latest coupon and price update (UTC):
Banggood
Banggood warehouse is located in Hong Kong. Well, there is one coupon available right now.
Link: Banggood product page
Coupon (184.00 USD): BGf8f0a0
(CouponsFromChina.com tip: 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
Old coupons (from the oldest to the newest): Terence, Tabby20, BG80a17d, BGa71544, BGc571a6, BGdd7800, BGb05cb8, BG880bef
(these codes were not working in latest coupon update, but you can check them out anyway)
Price history and price alert
Check out price history chart (for all/most countries) for POCO M4 Pro 4G 6/128GB.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).