The Prohibition
إِن تَتُوبَآ إِلَى ٱللَّهِ فَقَدْ صَغَتْ قُلُوبُكُمَا ۖ وَإِن تَظَـٰهَرَا عَلَيْهِ فَإِنَّ ٱللَّهَ هُوَ مَوْلَىٰهُ وَجِبْرِيلُ وَصَـٰلِحُ ٱلْمُؤْمِنِينَ ۖ وَٱلْمَلَـٰٓئِكَةُ بَعْدَ ذَٰلِكَ ظَهِيرٌ
If you two [wives] repent to Allah, [it is best], for your hearts have deviated. But if you cooperate against him - then indeed Allah is his protector, and Gabriel and the righteous of the believers and the angels, moreover, are [his] assistants.
Nearby ayahs help place verse 66:4 in sequence and meaning.
قَدْ فَرَضَ ٱللَّهُ لَكُمْ تَحِلَّةَ أَيْمَـٰنِكُمْ ۚ وَٱللَّهُ مَوْلَىٰكُمْ ۖ وَهُوَ ٱلْعَلِيمُ ٱلْحَكِيمُ
Allah has already ordained for you [Muslims] the dissolution of your oaths. And Allah is your protector, and He is the Knowing, the Wise.
وَإِذْ أَسَرَّ ٱلنَّبِىُّ إِلَىٰ بَعْضِ أَزْوَٰجِهِۦ حَدِيثًا فَلَمَّا نَبَّأَتْ بِهِۦ وَأَظْهَرَهُ ٱللَّهُ عَلَيْهِ عَرَّفَ بَعْضَهُۥ وَأَعْرَضَ عَنۢ بَعْضٍ ۖ فَلَمَّا نَبَّأَهَا بِهِۦ قَالَتْ مَنْ أَنۢبَأَكَ هَـٰذَا ۖ قَالَ نَبَّأَنِىَ ٱلْعَلِيمُ ٱلْخَبِيرُ
And [remember] when the Prophet confided to one of his wives a statement; and when she informed [another] of it and Allah showed it to him, he made known part of it and ignored a part. And when he informed her about it, she said, "Who told you this?" He said, "I was informed by the Knowing, the Acquainted."
عَسَىٰ رَبُّهُۥٓ إِن طَلَّقَكُنَّ أَن يُبْدِلَهُۥٓ أَزْوَٰجًا خَيْرًا مِّنكُنَّ مُسْلِمَـٰتٍ مُّؤْمِنَـٰتٍ قَـٰنِتَـٰتٍ تَـٰٓئِبَـٰتٍ عَـٰبِدَٰتٍ سَـٰٓئِحَـٰتٍ ثَيِّبَـٰتٍ وَأَبْكَارًا
Perhaps his Lord, if he divorced you [all], would substitute for him wives better than you - submitting [to Allah], believing, devoutly obedient, repentant, worshipping, and traveling - [ones] previously married and virgins.
يَـٰٓأَيُّهَا ٱلَّذِينَ ءَامَنُوا۟ قُوٓا۟ أَنفُسَكُمْ وَأَهْلِيكُمْ نَارًا وَقُودُهَا ٱلنَّاسُ وَٱلْحِجَارَةُ عَلَيْهَا مَلَـٰٓئِكَةٌ غِلَاظٌ شِدَادٌ لَّا يَعْصُونَ ٱللَّهَ مَآ أَمَرَهُمْ وَيَفْعَلُونَ مَا يُؤْمَرُونَ
O you who have believed, protect yourselves and your families from a Fire whose fuel is people and stones, over which are [appointed] angels, harsh and severe; they do not disobey Allah in what He commands them but do what they are commanded.