Since an employee is monthly rated, he or she should get paid whether for working days or for off days. I would use simple calculation like:
(4000/28) x 25 = 3571.41 whereas their calculation comes to only (4000/22)x18= 3272.73. There is some difference,
To cross check, for the first 3 days, it comes to (4000/28) x 3 = 428.59. So 4000-428.59 =3571.41 . Should be correct, isn't it?