All members of such board shall be citizens and actual residents of the cities in which they are appointed, except in the city of Niagara Falls where such members shall be citizens and actual residents of the county wherein the city is situated, the city of Middletown where such members shall either be citizens and actual residents of the county wherein the city is situated or anywhere within a twenty-five mile radius of such city within this state, and except in the city of Corning where such members shall be citizens and actual residents of the county wherein the city is situated or in an abutting or adjacent county, and except in the city of Rochester where such members shall be citizens and actual residents of the county wherein the city is situated, and except in the city of Watertown where such members shall be citizens and actual residents of the county wherein the city is situated. Provided however, in any city with a population of less than twenty-five thousand, or in any city which has less than four master plumbers, such members may be residents of the county or counties wherein the city is situated.