In the third verse, the text here is "See where He bows His sacred head." There's a general descent in the melody, and this mirrors that "bow[ing]."
In the fourth verse, the text is "O Lamb of God, was ever pain" (the sense is completed in the following line: "Was ever love, like Thine?"). "Pain" is sung to an accidental (F#), and this gives something of a sense of that discomfort. Additionally, that this accidental is a sharp in a flat key (G minor) may further emphasize this point.
