SALEM – A Malden man who went on a spree robbing area banks to feed his heroin addiction last year was sentenced to serve not less than four years and not more than six years in state prison Monday.Daniel Clark, 41, last known address of 1036 Main St., Malden, changed his not guilty plea to guilty Monday afternoon in Salem Superior Court confessing to five counts of unarmed robbery before Judge David A. Lowy.In addition to the state prison term, Clark will be on probation for another three years when he is released.The charges involve two unarmed robbery crimes out of Saugus and Salem.Three other charges stem from bank holdups in Melrose, Waltham and Cambridge last year.