For anyone inclined to poetic architecture, this is an example of commonality and contrast; see my article on the topic if you are interested. The four lines have a common theme, marked by 'before' and the shared rhyme; the first three lines share a subtheme, joined by 'shall'. The fourth line does not have this subtheme, and so does not have 'shall'. But it does have a transition, marked by the 'this' at the end of one phrase adjacent to the same word at the start of the next. For a related example, see When the Shadow's Left, and Not the Man, in which the common theme in the first stanza is marked by a shared word, and the different theme in the second stanza is marked by a different shared word.