Yes employer can revise the policies from time to time as it might be mentioned in your Appointment, however it should be informed to all employees prior to effect it. As you have completed 4.11 years I would suggest you to complete the balance notice period and get relieved properly. You are very much eligible for gratuity provided if you complete 5 years of continuous service. At the time of reliving make a request for your employer for gratuity in writing and your employer is bound to clear it within 30 days from your date of relieving. Else you can take up the matter with Assistant Commissioner of Labour of your jurisdiction. Holding salary after resignation is also not right as per law.You may represent to them to release salary for your day to day expenses.