There are a number of rights for landlords in Massachusetts. If the tenant has left the apartment, with his belongings without any notice and has not paid the rent, you are legally allowed to enter the apartment and take its control. But if the tenant has given you the rent for these 4 months and beyond, You are not allowed to enter the apartment. Here is a link that has every details about your right as a Landlord in Massachusetts:
www.malawforum.com