John Lightfoot on John 4:18

[Thou hast had five husbands, &c.] Christ stops her fleering mouth with the dung of her own unchaste conversation, charging her with that infamous sort of life she had hitherto lived: q.d. "Thou, for thy impudent adulteries, hast suffered divorce from five husbands already; and he whom thou now hast is not thy husband, but an adulterer."

The Cuthites do not understand the law about betrothings and divorcings. They had their customs of affiancing and divorcing; and perhaps by how much the less accurate they were about their divorces, (I mean with respect to the Jewish rules,) the nearer they might come to the first institution of Moses, who allowed no divorces but in the case of adultery. That this woman was dismissed from her husbands for these infamous faults of hers, seems evident, partly, from the extraordinary number of husbands, partly, that our Saviour mentions her husbands, as well as him that then lived adulterously with her: as if he would intimate, that she lived dishonestly under her husbands, as well as with this man.