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]
Title: The Plantagenet Ancestry, by William Henry Turton, 1968
Title: Ancestral Roots of Certain American Colonists, 7th Edition, by Frederick Lewis Weis, additions by Walter Lee Shippard Jr., 1999 194-7,Page: 127-29
The third shield has—Ferrers of Chartley, vairy or, and gules, impaling Earl of ‘Chester. Azure three garbs or. The inscription is: “ William of Ferrers Earle of Derby, maried Agnes, Lady of Chartley, daughter of Hugh Keviliok, Earle of Chester.” 
William de Ferers, 4th Earl of Derby, was born about 1162. 
William de Ferrers was the son of William de Ferrers, Earl of Derby, and his wife Sibyl de Briouse, who died 22 Sep 1247. 
1190 Earl of Derby
He succeeded his father in 1190 as Earl of Derby. 
1192 Marriage to Agnes/Alice of Chester
In 1192 he married in Agne [Alice] of Chester, daughter of Hugh Earl of Chester & his wife Bertrade de Montfort, who died 2 Nov 1247). The Annales Londonienses record that "Ranulphus comes Cestriæ" had four sisters, of whom "tertia…Agnes" married "comiti de Ferrariis, id est Derby, Willelmo seniori". 
Agnes was Lady of Chartley, Staffordshire, and Bugbrooke, Northamptonshire, following her brother's death in 1232
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. 
Arms on Behalf of King Richard
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.
"Staff. W., Earl of Ferrars was summoned to answer the complaint of Brian de l'Isle, that he had deforced him of one hundred solidates of land in Barton, of which Richard de l'Isle, brother of Brian, whose heir he is, died seised, &c. The Earl appeared by Robert de Cardoil, his attorney, and gave up the land. m. 16, dorso." 
Roll No. 39.
"Suff. William de Ferrars, the Earl, sued Richard, Abbot of St. Eadmund for the manor of Barton, belonging to his manor of Hegham, and in which the Abbot has no ingress except through Robert de Hese, who held it as Bailiff of King John, when the said manor was in the hands of the King's father; and he claimed by the seisin of his ancestor William Peverel, in the reign of King Henry, grandfather of the present King. The Abbot prayed a view, and the suit is adjourned to Easter; a view to be made in the interim. m. 14." 
Roll No. 5, Tower Records.
"Northt. Staff. Richard de Venables, Norman Pauntof, Robert de Touke and Robert de Staunton, four Knights sent to hear whom Agnes, wife of W. de Ferrars, Earl of Derby, attorned in her place in the plea Coram Reqe between Hugh de Albini and Hawise de Quency, and their coparceners, plaintiffs, and John, Earl of Chester and Huntingdon, deforciant, of the reasonable portion belonging to Hugh, Hawise, and the others, of the inheritance of Ralph formerly Earl of Chester, appeared in Court, and stated she had put in her place Thomas de Stanes or John Morel. m. 14." 
"North. John, Earl of Chester and Huntingdon, was summoned to answer the complaint of Hugh de Albini, William, Earl of Ferrars, and Agnes his wife, and Hawise de Quency, Countess of Lincoln, that he had deforced them of their reasonable share of the inheritance of Ralph, formerly Earl of Chester, and of which Ralph had died seised in the county of Chester, taking into account the shares they as well as John had received in other parts of the same inheritance; and they say that the said Earl holds the capital messuage in Chester, and Hugh de Albini has Coventry, with other lands; William de Ferrars and Agnes his wife have Certeslegh (Chartley), with other lands; and Hawise de Quency has Bullingbrock, with other lands." 
"In 1199 William de Ferrers, fourth Earl of Derby, son of William and grandson of Robert, second Earl of Derby, purchased for 2,000 marks from King John the manor, hundred and park of Higham Ferrers and certain other lands, at the same time relinquishing what claim he had through his grandmother, Margaret Peverel, to the other lands of William Peverel. (fn. 86) William de Ferrers died in 1247 and was succeeded by his son William fifth Earl of Derby." 
Hundred of West Derby
"Henry III on 18 October, 1229, granted all the land between Ribble and Mersey, including the vill of West Derby with the wapentake and the forest, the borough of Liverpool, the vill of Salford with the wapentake, and the wapentake of Leyland, to Ranulf, earl of Chester and Lincoln, to hold in fee by rendering yearly at Michaelmas a mewed goshawk or 40s. (fn. 14) The assized rent of the demesne, with the service of the tenants holding in thegnage and at fee farm, and sake fee of the military tenants within the hundred, then amounted to £46 16s. 2d. (fn. 15) Upon the earl's death, in 1232, without issue this fee descended to William de Ferrers, earl of Derby, in right of Agnes his wife, one of the sisters and co-heirs of the earl of Chester. (fn. 16)
The earl died in 1247, (fn. 19) having predeceased his wife but a few weeks. That he was the builder of Liverpool Castle may be inferred from writs of 19 January, 1235, for an aid to be made to him for the strengthening of his castle of Liverpool, (fn. 20) and of 10 November, 1247, directed to the escheator beyond Trent to deliver to William de Ferrers the lands which had been Agnes de Ferrers', and the castles of West Derby and Liverpool. (fn. 21)" 
Peveril, Bolsover, and Horston
"Further north, the earl of Chester was granted the castle at Newcastle-under-Lyme, and Horston Castle, in Horsley north of Derby, was released to William II de Ferrers, earl of Derby, so that he could keep his wife, a sister of earl Ranulph of Chester, in safety there." 
"William de Ferrers was forced to hand the castle, along with Peveril and Horston Castle back to King Henry III in the 1220s. The family's resentment at this treatment led to the Earls of Derby lending their support to the Provisions of Oxford which are often regarded as England's first written constitution." 
Walton and Hale
"The latter King [Henry III] , early in his reign, bestowed the land between Ribble and Mersey upon Ranulf, Earl of Chester, whose title passed to William de Ferrers, Earl of Derby, in right of his wife Agnes, daughter and co-heir of the said Earl of Chester, and from him to his son William, Earl of Derby, who died in 1254, and so to Robert de Ferrers, son and heir of the last-named Earl. The said Robert was attainted of high treason for rebellion, but was pardoned and his estates restored; but rebelling again, his estates were confiscated, those in Lancashire being ultimately given to Edmund, Earl of Lancaster." 
“Willielmus comes de Ferrariis, filius Willielmi comitis de Ferrariis” donated property to Tutbury Priory, for the soul of “Agnetis uxoris meæ”, by undated charter. 
"Robert de Ferrars, the last Earl of Derby, confirmed all the benefactions of his ancestors by charter, but by him the priory was pulled down in the year 1260, and was not rebuilt till theyear 1307, no doubt by the Earl of Lancaster." 
↑ Sir Bernard Burke, Dormant, Abeyant, Forfeited, and Extinct Peerages, Burke's Peerage, Ltd., London, 1883, p. 196, Ferrers, Earls of Derby
↑ 6.06.16.26.3 "Plea Rolls for Staffordshire: 1231-39," in Staffordshire Historical Collections, Vol. 4, ed. George Wrottesley (London: Staffordshire Record Society, 1883), 81-89. British History Online, accessed March 23, 2017, .
↑ "The borough of Higham Ferrers," in A History of the County of Northampton: Volume 3, ed. William Page (London: Victoria County History, 1930), 263-279. British History Online, accessed March 16, 2017, .
↑ Grant to William de Ferrars, earl of Derby, of Higham Ferrers and other lands in co. Northants 
↑ "Hundred of West Derby: Introduction," in A History of the County of Lancaster: Volume 3, ed. William Farrer and J Brownbill (London: Victoria County History, 1907), 1-4. British History Online, accessed March 16, 2017, .
↑ The Magna Carta Project, The Itinerary of King John, 17 May 1215 - 23 May 1215 
↑ "Appendix: Additional material to fines," in Final Concords For Lancashire, Part 1, 1189-1307, ed. William Farrer (Edinburgh: Record Society of Lancashire and Cheshire, 1899), 216-219. British History Online, accessed April 3, 2017, .