In Book 3 we saw Augustine’s fall away from the Church, in Book 5 we will see the beginning of his return. Book 4, however, is focused primarily on his years as a Manichean.
This is where we get the fairly brief description which is nearly all we have on Augustine’s longest romantic relationship:
In those days I lived with a woman, not my lawful wedded wife but a mistress whom I had chosen for no special reason but that my restless passions had alighted on her. But she was the only one and I was faithful to her. Living with her I found out by my own experience the difference between the restraint of the marriage alliance, contracted for the purpose of having children, and a bargain struck for lust, in which the birth of children is begrudged, though, if they come, we cannot help but love them.
We also hear a bit about Augustine’s life as a hot shot young rhetorician. In addition to his Manichean beliefs, he falls into consulting astrologers frequently, in part to learn the auspices when he’s entering major academic competitions. At one point, a magician of some sort offers to assure that he will win a competition, but although Augustine finds the idea that that stars and planets can influence worldly events appealing (and has no qualms about consulting astrologers and books of astrology) he recoils at the idea of the magician sacrificing animals to dark powers in an attempt to secure a victory for him.