What does this rather vocative phrase from Romans 10:4 mean? What relationship between Christ and the Law is Paul envisioning at this point in his letter to the Roman church?

To help answer this question it can be helpful to read Romans 10:2-4 alongside Romans 3:20-22. *This suggestion is from Lionel Windsor. You can find a more detailed blog here.

The Law's purpose (as Romans 10:4 describes it) was to point beyond itself to Jesus as the only one who could actually establish God’s righteousness.

Jesus isn't the "end" of the law in the sense of declaring its irrelevance or his being an updated version of it; Law 2.0, so to speak. Jesus isn’t the culmination of the Law simply in that he perfectly embodied it himself.
In witnessing to Israel’s sin (Rom 7:7) the Law (along with the prophets) had always borne witness to the fact that God’s righteousness would be revealed only through faith.

Jesus is the end or culmination of the law in the sense that he’s the one the Law (and the prophets) had been directing Israel's attention to all along; the one through whom the gift of God’s Righteousness is graciously given.

This is not to imply that Romans 10:4 is everything that the scriptures (nor Paul himself) have to say about the Law. Yet hopefully, this helps us grasp something of Paul’s meaning at this particular point in Romans’ flow of thought.