I shut down all Amazon RDS instances but it still shows service is running and it charged me 4 days usage even I haven't connected or used any data?

AWS Billing is performed on a monthly basis but you accrue costs throughout each day. We are in day 21 of March. If the RDS was running in the last 21 days and not in the free tier, then you will be charged for the usage of RDS even if it was running for 1 hour. You will be billed for that usage at the end of the month