All of the music-related things I noticed occur in the same phrase, although it appears multiple times throughout the hymn:
The first two lines of the hymn are "The will of God is always best / And shall be done forever." The second of those lines is sung to the phrase above, and "forever" is sung with a melisma (A G F# G), musically giving a sense of duration.
The first two lines of the fourth verses are "When life's brief course on earth is run / And I this world am leaving." Again, the second of those lines is sung to the phrase above, and "leaving" is sung with a melisma (G F# G) for a sense of movement (although it's a bit more metaphorical).
In the third verse, the third and fourth lines are "When Satan sorely troubles me, / Then do not let me waver." Here, "waver" is sung with that same melisma (G F# G), so while it's negated, there's a musical sense of its meaning.
In looking over the hymn again in order to write this post, I noticed that this phrase has a cross inscription. I think its only connection to the text appears at the very end of the hymn (and while the lines here occupy the same musical space as those quoted above, there's an additional line break): "From sin and hell, / And death as well, / By You the victo'ry reaping." That last line is what's sung to the phrase above, and the cross inscription corresponds to "You the vict'ry." It is through Christ's crucifixion and resurrection that we have that victory.
Previously, I'd also noticed a night/day merism at the end of the second verse: "By night and day / God is my stay; / He never sleeps nor slumbers." This is drawn from Psalm 121 and even maintains the alliteration ("sleeps nor slumbers") which is present in all of the English translations I referenced and which provides a sense of the constancy of God's care.
