Jacob narrows the future focus in Isaiah. Isaiah’s more general promises become a very specific future event. Isaiah stressed that the covenant with Israel was still in place despite Israel’s hardships. Jacob makes that covenant into a promise of the Atoning Messiah’s mortal mission.
Verse 3 therefore looks back to Isaiah and emphasizes the blessings that will come because of the covenantal relationship, while verse 4 sets the context for the Messiah’s sacrifice.