https://penelope.uchicago.edu/Thayer/E/Roman/Texts/Quintilian/Institutio_Oratoria/1B*.html
LacusCurtius • Quintilian — Institutio Oratoria — Book I, Chapters 4‑6
Part of a complete English translation. Very large site on ancient Rome contains many Greek and Latin texts, translations and related material.
book iquintilianoratoriachapters