If the petition accompanying the proposed ordinance is signed by a number of registered voters of the city equal to at least twenty percent (20%) of the entire vote cast for all candidates for mayor at the last preceding general election at which a mayor was elected, and contains a request that the proposed ordinance be submitted to a vote of the registered voters at a special election, the council shall within twenty (20) days thereof either:
(a) Pass the ordinance without alteration, subject to the referendum provided by this charter, or
(b) Call a special election to be held within ninety (90) days from the date of the certificate of the city clerk attached thereto, and at such special election, the proposed ordinance shall be submitted without alteration to the vote of the registered voters of the City. Provided, however, that if any election is to be held in the city within four months from the date of the certificate by the city clerk, then the ordinance shall be submitted at the election, and no other special election for the purpose of submitting the ordinance shall be called.