"Therefore" is a conjunction that bridges either to a command, a conclusion, an exhortation or perhaps, all three. Here it connects to a statement that has elements of all three, with exhortation being the strongest. Paul is appealing to us to make practical application of the things that immediately preceded the word "therefore."
He is referring to what appeared in the first three chapters, as they lay the foundation for what follows in chapter 4 and beyond, that is, what is necessary to produce unity. It does not matter whether it is in a family, church, government, or nation. Ephesians 5, for example, speaks of the unity of husband and wife, which sets the pace for the family and prepares for the Kingdom of God and unification with Jesus Christ for all eternity.
That the chapter breaks in Ephesians 4:1 is almost a disservice. Paul did not write it that way; he wrote it as a free-flowing letter from beginning to end. One thought flows right into another. If we would read it that way with understanding, it would have greater impact than it does in this form. Nevertheless, we tend to segment things because we desire order, so breaking it up like this makes possible a concordance and finding words and phrase relatively easy. Even so, sometimes those "helps" are misleading and a disservice. The break in continuity might have worked better elsewhere.
We need to understand that, with that word "therefore," Paul is telling us to do something! In light of all that appears in the first three chapters of this book, which provides the doctrinal base for unity, we ought to do something. If we believe it and put it into practice, we can have unity! But if we fail in this, unity will also fail.
So we need the guidance of the doctrines to know the direction to take and what to do along the way. But, if we do not go in that direction and do not do what it says, we have not listened to the "therefore." It is our spiritual business to put into practical application the doctrines explained at the beginning of the book.
John W. Ritenbaugh
Unity (Part 3): Ephesians 4 (A)