John Seymour, 4th Duke of Somerset and 3rd Marquess of Hertford (before 164629 April 1675) was an English peer and MP.
Life
He was a younger son of William Seymour, 2nd Duke of Somerset, and Lady Frances Devereux. He was the only one of the second duke's sons still alive when his father died and the title descended to his six-year-old nephew, William.
In 1661 he married Sarah, daughter and co-heiress of Sir Edward Alston and widow of George Grimston
He was elected Member of Parliament for Marlborough in 1661. Unhappy in his marriage and having fallen out with his nephew's guardian and his mother, he entered Gray's Inn in 1666 in order to escape his creditors.