Chapter 4 argues that the family is not just where love is enjoyed but where it is patterned. We learn to treat elders like grandparents, peers like siblings, and younger people like children. In other words, the home gives us the emotional grammar for the rest of society.
That is why the chapter calls family a microcosm of the world and the cosmos. It is not because the family contains everything, but because it trains the relational forms through which everything else is to be handled.
This matters for church and civic life. A person who has never learned to widen family-pattern love outward will keep treating strangers as abstractions. A healthy household trains the imagination to recognize kin-like obligations outside the household.