Any city having a population of less than 5,000 inhabitants may be divided into 3 wards. If it contains a population of 5,000 or upwards it may be divided into 4 wards, and an additional ward for every additional 2,000 inhabitants above 5,000 and up to 10,000. But any city having, at the time of its being brought under or subject to the provisions of this act, a greater number of wards in proportion to its population than above mentioned, shall not be required to diminish the number of its existing wards.