Hi @mvjor
Have you tried connecting a different known working compatible charger or USB-C charger cable to the speaker to see if it charges OK, you didn't say?
if it still doesn't indicate that it is charging then the speaker will have to be opened and inspected to find out what's wrong e.g. loose charge port in speaker, faulty motherboard, maybe even faulty battery
Here's the ifixit [[Topic:JBL Charge 4]] guide that will help to gain access to the motherboard etc.