The governing body of a city operating under the modern council form of government is the city council, which is composed of not less than four members, one of whom is the mayor, all elected at large or by wards. Candidates for the council shall run for either mayor or council member but not both at the same time. The terms of members of the council is four years, commencing on the first of July of the year in which the member was elected and until their successors are elected and qualified. However, the council shall establish by ordinance a procedure whereby one-half of all council members, as nearly as is practicable, are elected biennially. The number of council members may be increased or decreased pursuant to section 40-06-09.