In all cities and towns under special charter, located in counties of the first class under a charter form of government, which have attained a population of more than thirty thousand inhabitants but less than two hundred fifty thousand inhabitants subsequent to the granting of their special charter, and every four years thereafter, there shall be elected a mayor, police judge and two councilmen from each ward, who shall hold their respective offices for a term of four years and until their successors are elected and qualified.