Albert Barnes Commentary John 4:18

"for thou hast had five husbands; and he whom thou now hast is not thy husband: this hast thou said truly." — John 4:18 (ASV)

Hast had five husbands. Who have either died; or who, on account of your improper conduct, have divorced you; or whom you have left improperly, without legal divorce. Either of these might have been the case.

Is not thy husband. You are not lawfully married to him. Either she might have left a former husband without divorce, and thus her marriage with this man was unlawful, or she was living with him without the form of marriage, in open guilt.