Thank you for the hint with AliExpress.
Concerning the voltage:
I’m not saying the charger is dangerous nor safety would be at risk for the battery. Instead, the battery will be worn down quickly for pushing it to/beyond its specifications. It’s not only the 4.21V but also the extreme long CV phase that is applied to the battery. Usually, voltage drops from 4.20V to 4.14-4.16V when charging is finished as the termination phase is rather short.