'Registers an Amazon RDS instance with a stack.
**Required Permissions**: To use this action, an IAM user must
have a Manage permissions level for the stack, or an attached
policy that explicitly grants permissions. For more
information on user permissions, see `Managing User
Permissions`_.
:type stack_id: string
:param stack_id: The stack ID.
:type rds_db_instance_arn: string
:param rds_db_instance_arn: The Amazon RDS instance\'s ARN.
:type db_user: string
:param db_user: The database\'s master user name.
:type db_password: string
:param db_password: The database password.'

def register_rds_db_instance(self, stack_id, rds_db_instance_arn, db_user, db_password):

    params = {'StackId': stack_id, 'RdsDbInstanceArn': rds_db_instance_arn, 'DbUser': db_user, 'DbPassword': db_password}
    return self.make_request(action='RegisterRdsDbInstance', body=json.dumps(params))
