Mahaut (1150-Dec. 17, 1192), the only surviving child of the Duke Robert de Hauteville became the countess of Grignon “is said to have killed four husbands.”
She was married four times: first to Eudes II Seigneur d'Issoudun who died two years after the marriage, second Guy I, Count of Nevers and Tonnerre with whom she was married 8 years. When he refused to do homage to her cousin Hugues III he was imprisoned and died a year later. She inherited the county of Tonnerre from him. Her third husband was Pierre of Flanders. She married her last husband Robert II Count of Dreux around 1178 but the marriage was annulled due to consanguinity. After that she retired to Fontevraud and became a nun.
[Paul D. Abbott, Provinces, Pays, and Seigneuries of France, 1981, p. 203]
For links to other cases of woman who murdered 2 or more husbands (or paramours), see Black Widow Serial Killers.