A.T. Robertson Commentary 1 John 4:9

"Herein was the love of God manifested in us, that God hath sent his only begotten Son into the world that we might live through him." — 1 John 4:9 (ASV)

Was manifested (εφανερωθη). First aorist passive indicative of φανεροω. The Incarnation as in 3:5. Subjective genitive as in 2:5.

In us (εν ημιν). In our case, not "among us" nor "to us." Cf. Galatians 1:16.

Hath sent (απεσταλκεν). Perfect active indicative of αποστελλω, as again in verse 14, the permanent mission of the Son, though in verse 10 the aorist απεστειλεν occurs for the single event. See Joh 3:16 for this great idea.

His only-begotten Son (τον υιον αυτου τον μονογενη). "His Son the only-begotten" as in Joh 3:16. John applies μονογενης to Jesus alone (John 1:14,18), but Luke (Luke 7:12; Luke 8:42; Luke 9:38) to others. Jesus alone completely reproduces the nature and character of God (Brooke).

That we might live through him (ινα ζησωμεν δι' αυτου). Purpose clause with ινα and the first aorist (ingressive, get life) active subjunctive of ζαω. "Through him" is through Christ, who is the life (John 14:6). Christ also lives in us (Galatians 2:20). This life begins here and now.