I am creating two object(Account-Case) with master-details relationship. Now Business requirment is like this: The record in these object should be in Ratio (1:4). Like 1 Record of the Master Object should be 4 record of the Child Object (PS: Not more than 4).

And now the record (data) will be store in MONTH wise. Like each month should only store 4 record. The user should get an error when he try to insert 5^th record in any month at any time. He can modify or delete the record but he cannot add 5^th record at any month. Each month should only contain 4 record.
 

If anyone has any suggestion to this problem of Ratio (1:4) and month-wise record. Pls Suggest.
PS: Only required answer through Validation rule and Flow. Doesn’t required of triggered base answer.