Aliexpress
Here are coupons that you can use.
Latest price update (UTC time):
Banggood
Banggood warehouse is located in China. There is one great coupon!
Link: Banggood product page
Coupon (28.99 USD): BANGGOODSD
(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): 2019FG, 2019fg, BGF1111, fInbG2020, BGff2020, BCNLXK, BGSM30%, BG1128, BG10OFFAFF1, BGFLBA
(probably won’t work, but you can try them out)
Price history and price alert
Here is price history (for all/most countries) for 4x Panasonic NCR18650B Battery.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).