Theophylact of Ohrid

Through His teaching an even greater number of them believed. Although the Evangelist does not narrate in detail the wondrous words of His teaching, he gives us to understand the power of His divine teaching from the outcome of the matter. For the Evangelists omit much even of the great deeds, because they write not from motives of vainglory, but for the sake of truth. It is likely that while among the Samaritans the Lord also taught something divine, because they, without having seen any miracle, believe in Him and ask Him to stay. …

Commentary on John ↗

Irenaeus

But as many as feared God, and were anxious about His law, these ran to Christ, and were all saved. For He said to His disciples: "Go ye to the sheep of the house of Israel, which have perished." And many more Samaritans, it is said, when the Lord had tarried among them, two days, "believed because of His words, and said to the woman, Now we believe, not because of thy saying, for we ourselves have heard [Him], and know that this man is truly the Saviour of the world." And Paul likewise declares, "And so all Israel shall be saved;" …

Against Heresies Book IV, Chapter II ↗

Thomas Aquinas

Then (v 41), the Evangelist says that the fruit resulting from the witness of the woman was increased by the presence of Christ; and this in three ways. First, in the number of those who believed. Secondly, in their reason for believing. Thirdly, in the truth they believed. The fruit was increased as to the number of those who believed because while many believed in Christ on account of the woman, many more believed in him because of his own words, i.e., Christ's own words. …

Commentary on John ↗

Bonaventure

"And many more believed because of his word." Nor is this surprising, because, as Peter says to him below in chapter 6, "Lord, to whom shall we go? You have the words of eternal life." Nor did he only multiply, but also confirmed.

Commentary on John, Chapter 4 ↗