This nobleman, upon the return of King Richard from captivity, took arms in his behalf and, joining the Earl of Chester, besieged Nottingham Castle, which, after a brief resistance, surrendered. For this and other acts of fidelity, he was chosen by the king to sit with the rest of the peers in the great council held at the said castle in Nottingham in the ensuing March. Moreover, at Richard's second coronation he was one of the four that carried the canopy over the king's head. Upon the accession of King John, his lordship, with the Earls of Clare and Chester, and other great men, swore fealty to the new monarch but upon the condition that each person should have his right. His lordship was present at the coronation of King John and 7 June following, being solemnly created Earl of Derby by special charter dated at Northampton, he was girt with a sword by the king's own hands (being the first of whom in any charter that expression was used). He had also a grant of the third penny of all the pleas before the sheriff throughout the whole country whereof he was earl, to hold to him and his heirs as amply as any of his ancestors had enjoyed the same. Moreover, in consideration of 4,000 marks, he obtained another charter from the king of the manor of Higham-Ferrers, co. Northampton, with the hundred and park; as also of the manors of Bliseworth and Newbottle, in the same shire; which were part of the lands of his great grandfather, William Peverel of Nottingham. King John also conferred upon him a mansion-house situated in the parish of St. Margaret within the city of London, which had belonged to Isaac, a Jew, at Norwich, to hold by the service of waiting upon the king (the earl and his heirs), at all festivals yearly without any cap, but with a garland of the breadth of his little finger upon his head. These liberal marks of royal favour were felt so gratefully by the earl that in all the subsequent struggles between the king and the refractory barons, his lordship never once swerved from his allegiance, but remained true to the monarch; and loyalty to the interests of his son, King henry III. His lordship assisted at the coronation of the new monarch and immediately after the ensuing Easter, he took part with the famous William Marshall (governor of the king and kingdom), the Earls of Chester and Albemarle, and many other great men in the siege of Mountsorell Castle in Leicestershire, then held by Henry de Braybroke and ten other stout knights. And the same year was likewise with those noble persons at raising the siege of Lincoln, which place the rebellious barons with Lewis, King of France, had invested. His lordship m. Agnes, sister and one of the co-heirs of Ranulph, Earl of Chester, by whom he had two sons, William and Thomas. He died of the gout in 1246 and his countess d. in the same year after a union, according to some authorities, of seventy-five, and by others, of fifty-five years. His lordship was s. by his elder son, William de Ferrers, 5th Earl of Derby. [Sir Bernard Burke, Dormant, Abeyant, Forfeited, and Extinct Peerages, Burke's Peerage, Ltd., London, 1883, p. 196, Ferrers, Earls of Derby]
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There is substantial confusion over his name. See The Complete Peerage Vol. 4, p 193 for an account. Personally, I feel there could have been two brothers, William and Robert, Robert being the Earl and when he died at Acre his nephew William son of his brother William succeeded, but no documents support this theory either! In The Complete Peerage vol. XIV ,p .250 it is suggested that Robert is a fabrication by Vincent, Earl of Ferrieres. [Brian Tompsett, Directory of Royal Genealogical Data]