The focus of Book 4 is rhetoric and Augustine’s views on effective Christian teaching. Augustine begins by discussing how rhetoric is of central importance and admits that while it can occasionally be used for bad ends that does not mean Christians should not develop rhetorical skills to serve good ends. Rather, effective rhetorical practices can aid the spread and understanding of the Christian faith. Augustine urges his readers to read the speeches of famous orators as models for their own oratory.

Augustine also addresses the charge against canonical writers of the Bible. Critics claim that their style is clumsy and inelegant compared to the great writers of the Greek and Roman classical tradition. Augustine offers a firm and lengthy rebuttal by quoting passages of scripture that he believes demonstrate the power and efficacy of the style in scripture, especially the works of St. Paul. He also draws upon the writings of Church Fathers, such as Cyprian and Ambrose, as examples of effective rhetorical technique.

Augustine then lays out the precepts that all Christian teachers should follow to offer engaging and effective instruction in the faith.