If “the 4th century Councils of Rome and Carthage and the Synod of Hippo” were authoritative, then Trent would not have needed to take up the subject. Local councils were not binding. The Council of Trent was.
Church history is an area that I have a fair amount of knowledge of. The Council of Trent was the 19th Ecumenical Council of the Church. It wasn't a single session or a single topic deal. It took place over a 12 year period.
Trent was hardly the first Ecumenical Council to address the Canon. The main purpose of the Council was to rebut and condemn the principles and doctrines of Protestantism and to clarify and reaffirm the doctrines of the Catholic Church on all disputed points. I think it accomplished this quite well.