God sent His Son (4:4–5)

Says “when the time had fully come, God sent His Son, born of a woman, born under the law” (ὅτε δὲ ἦλθεν τὸ πλήρωμα τοῦ χρόνου (hote de ēlthen to plērōma tou chronou, “when the fullness of the time came”); ἐξαπέστειλεν ὁ θεὸς τὸν υἱὸν αὐτοῦ (exapesteilen ho theos ton huion autou, “God sent forth his Son”); γενόμενον ἐκ γυναικός, γενόμενον ὑπὸ νόμον (genomenon ek gynaikos genomenon hypo nomon, “born of a woman, born under the law”)). The purpose is “to redeem those under the law, that we might receive our adoption as sons” (ἵνα τοὺς ὑπὸ νόμον ἐξαγοράσῃ (hina tous hypo nomon exagorasē, “in order that he might redeem those under the law”); ἵνα τὴν υἱοθεσίαν ἀπολάβωμεν (hina tēn huiothesian apolabōmen, “in order that we might receive the adoption as sons”)). The wording, including υἱοθεσία, is treated in The fullness of time, and born of a woman (4:4–5) and υἱοθεσία.

Implies God sends a Son who is already his Son, and the two participles place him in a true human birth and under the law, in order to redeem those under the law. Adoption and the Spirit’s witness are also treated in Led by the Spirit: adoption, "Abba! Father!", the Spirit's witness, and heirs with Christ (Romans 8:14–17) and τὸ δὲ πνεῦμα ζωή … πνεῦμα υἱοθεσίας … Αββα ὁ πατήρ (8:9–17).