Regular meetings of the city council of a city of the primary class shall be held at least once each week on such days and at such times as the city council may prescribe in its rules, and special meetings shall be held whenever called by the mayor or any four members of the city council. The city council may choose not to meet during any week in which a federal or state holiday occurs. Four members of the city council shall constitute a quorum for the transaction of any business, and four affirmative votes shall be required to pass any measure or to transact any business unless it is otherwise provided by any home rule charter of a city of the primary class.