Verses 3 and 4 mark the transition from Isaiah's text of promises into a specific future promise. While Isaiah's text emphasized the covenant with Israel as still being in place in spite of Israel's hardships, Jacob will take that covenant further, and extrapolate the covenant into the promise of the coming of the Savior.
Verse 3 therefore looks back to Isaiah and emphasizes the blessings that will come because of the covenantal relationship, and verse 4 sets up the question of the ultimate atoning sacrifice of the Savior.