Section 4. The commissioner shall appoint, subject to appropriation, such other assistant, deputy or associate commissioners as the commissioner may determine to be necessary or desirable to carry out the work of the department. Assistant, deputy and associate commissioners shall be assigned areas of responsibility to be specified by the commissioner. At the time of appointment, they shall possess such qualifications as the commissioner shall determine. Persons appointed to such positions shall serve at the pleasure of the commissioner. Chapter thirty-one and section nine A of chapter thirty shall not apply to said positions.