John Seymour, 4th Duke of Somerset

John Seymour, 4th Duke of Somerset and 3rd Marquess of Hertford (before 1646 – 29 April 1675) was an English peer and MP.

Life
He was the only surviving son of William Seymour, 2nd Duke of Somerset, and Lady Frances Devereux. He married Sarah, daughter and co-heiress of Sir Edward Alston and widow of George Grimston in exchange for a dowry of £10,000. Her father negotiated an independent income of £300 per year for his daughter and additional clauses to cover the possibility that she became a widow. The marriage was a financial success but otherwise it was a disaster. William liked drinking and gambling, but Sarah did not.

He was elected Member of Parliament for Marlborough in 1661 and entered Gray's Inn in 1666. He succeeded his nephew as the 4th Duke of Somerset in 1671.

He spent the last two separated from his wife after she petitioned the King concerning his behaviour before he died in 1675, childless. He was buried in Salisbury Cathedral. He was succeeded in the dukedom by his cousin, Francis Seymour. The Marquessate of Hertford became extinct on his death.