Section 17C. Upon the filing with the city clerk of a petition, which petition shall be subject to the provisions of section seven or section seven A of chapter fifty-three, signed by at least five per cent of the number of registered voters residing in the city at the last regular city election, the city clerk shall place upon the ballot for the next regular city election to be held not less than sixty days after the date of the filing of such petitions the following question:?
''Shall the term of office of mayor of the city of
___ be four years?''
If a majority of the votes cast in answer to said question is in the affirmative, the term of office of the mayor of said city shall thereafter be for four years and until the election and qualification of his successor, beginning with the next regular city election following the acceptance of this question.
This section shall apply only in cities which have adopted Plan A, Plan B or Plan F under this chapter and in cities which have, under the provisions of any special act or any charter adopted under the provisions of Article LXXXIX of the Amendments to the Constitution, a mayor, as defined in subsection (a) of section ten of chapter forty-three B.
The provisions of this section shall be applicable notwithstanding the provisions of section eighteen of said chapter forty-three B. Nothing contained herein shall be construed to prevent the amendment of a city charter by any method available under said Article LXXXIX of the Amendments to the Constitution or under said chapter forty-three B.