For legislative purposes, cities shall be classified as follows based upon population as ascertained by the most recent Federal decennial census:
a. First class--cities having a population of more than 150,000;
b. Second class--cities having a population of not less than 12,000 but not more than 150,000;
c. Third class--all cities which are not first- or second-class cities except cities bordering on the Atlantic ocean being seaside or summer resorts;
d. Fourth class--cities bordering on the Atlantic ocean which are seaside or summer resorts.