Section 4. In each city which is not exempt by the provisions of section ten there shall be a licensing board appointed by the mayor, subject to confirmation by the board of aldermen or, if there is no such board, by the city council, consisting of three persons, who shall not be engaged, directly or indirectly, in the manufacture or sale of alcoholic beverages, who have been residents of the city in which they are appointed for at least two years immediately preceding their appointment. One member shall be appointed from each of the two leading political parties and the third member may also be appointed from one of said parties. If any member of said board engages directly or indirectly in such manufacture or sale, his office shall immediately become vacant.