This can be confusing because the chapter and verse numbering is different between English Bibles, the MT and LXX. The English verse Mal. 6:4 is somewhat different between MT and LXX, and the LXX has an extra verse after 6:4. The translations of the MT and LXX follow the texts.
And he will turn the hearts of fathers to their children and the hearts of children to their fathers, lest I come and strike the land with a decree of utter destruction.”
(Mal. 4:6, ESV)
וְהֵשִׁ֤יב לֵב־אָבוֹת֙ עַל־בָּנִ֔ים וְלֵ֥ב בָּנִ֖ים עַל־אֲבוֹתָ֑ם פֶּן־אָב֕וֹא וְהִכֵּיתִ֥י אֶת־הָאָ֖רֶץ חֵֽרֶם׃
(Mal. 3:24, MT)
He shall reconcile parents with children and children with their parents, so that, when I come, I do not strike the whole land with utter destruction.
(Mal 3:24, JPS Tanakh)
23 ὃς ἀποκαταστήσει καρδίαν πατρὸς πρὸς υἱὸν καὶ καρδίαν ἀνθρώπου πρὸς τὸν πλησίον αὐτοῦ, μὴ ἔλθω καὶ πατάξω τὴν γῆν ἄρδην. 24 μνήσθητε νόμου Μωυσῆ τοῦ δούλου μου, καθότι ἐνετειλάμην αὐτῷ ἐν Χωρηβ πρὸς πάντα τὸν Ισραηλ προστάγματα καὶ δικαιώματα.
(Mal. 3:23–24, LXX)
who will restore the heart of a father to a son and the heart of a person to his neighbor, lest I should come and strike the land entirely.
6 “Remember the law of my servant Moses, as I commanded him in Horeb with all Israel, with commandments and decrees.”
The Lexham English Septuagint (Second Edition, Mal. 4:5–6). (2020). Lexham Press.