Here we go again. Our refrain is repeated once more, only this time the speaker addresses a babe and not a maiden. And no, he's not talking about a cute girl. In this case babe just means baby.
If the speaker is talking to an infant, maybe he's going to talk about the infant's father. And maybe papa's in the war, like the young lover from stanza 1.
Because your father tumbles in the yellow trenches, Raged at his breast, gulped and died, Do not weep. War is kind.
Our hunch was correct: the speaker is talking about the babe's father.
He tells the little infant not to weep over the fact that his father is dying in some yellow trenches.
That death was pretty gruesome: he tumbled, then raged at his breast, then gulped, and finally croaked.
Okay, we need to do some unpacking here.
Yellow trenches? That's a strange description. It probably refers to the color of the dirt on the battlefield or of the trenches that soldiers would sometimes live in and fight from.
Yellow makes us think of sickness and death; the yellowing of one's skin caused by jaundice, for example, yellow fever, things like that.
Was the father angry at himself? No, this isn't what "raged at his breast" means. We're thinking he grasped his chest where he was wounded, or pounded it while in his death-throes.
Then he took one last gulp, or his final breath.
Note the strange tense changes in here. The lines start with that whole movie-scene effect we talked about in stanza 2, with the speaker talking in the present tense (tumbles).
But then he kind of zooms away from that in-your-face moment and narrates the event as if it was in the past: raged, gulped, died.
Maybe he's creating distance between his narration and the events, or maybe he's implying that the father continued to tumble after death.
Note that this stanza very closely mimics or imitates the poem's first stanza, with the refrain, and the structure, and all that jazz. It's a safe bet that the fifth stanza is going to be pretty similar.
The speaker concludes by reiterating that the babe shouldn't weep because war is kind, after all.