Hi Deepthi,

If you want to add 4 leaves to all the employee at every month end then best approach would be Batch Class.
You can write a batch class and schedule to every month end that will run at each month end automatically and perform the logic written inside the batch class.
below is the code sample for reference.

Global class CalculateLeaveBatch implements Database.Batchable<SObject>
{
    
    Global Database.QueryLocator Start(Database.BatchableContext BC)
    {
        string contactQuery = 'select id, firstname, lastname,  total_leaves__c from contact ';
        return Database.getQueryLocator(contactQuery);
    }
    
    Global void Execute(Database.BatchableContext BC,List<SObject> contactsToProcess)
    {
      list<Contact> lstCon = new list<Contact>();
      for(SObject conRecord : contactsToProcess)    
        {
            Contact con = (Contact) conRecord;
            con.total_leaves__c = con.total_leaves__c + 4; // add the leaves here
            lstCon.add(con);
        }
        if(lstCon.size() > 0){
            Update lstCon;
        }
    }
    
    Global void Finish(Database.BatchableContext BC)
    {
        // Post logic like to send email etc...
    }
}

you can refer the below documentation of batch class for more details
https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_batch_interface.htm