With the death of his mother Bertha in early 1156, Conan IV expected to inherit the ducal throne. However, he was denied his inheritance by his step-father Eudes, who refused to let go over his authority. Eudas may have entered into a pact with Hoel, Count of Nantes, to divide Brittany between them. But Hoel was under threat of rebellion in Nantes, sponsored by Geoffrey VI, Count of Anjou, and he could not send Eudas any aid. Within the year Conan IV was able to capture and imprison Eudas, and claim his inheritance.
By 1158, Geoffrey VI, Count of Anjou died and Conan seized Nantes, reuniting the Duchy once again. However, Geoffrey's brother King Henry II of England, responded by seizing the Earldom of Richmond, Conan's paternal inheritance, and demanded the return of Nantes. Conan and Henry made peace, and Conan married Henry's cousin, Margaret of Scotland, in 1160. Margaret was daughter of Henry of Scotland, 3rd Earl of Huntingdon and Ada de Warenne, a daughter of William de Warenne, 2nd Earl of Surrey and Elizabeth de Vermandois.
Conan faced several revolts from his own nobles, rebellions possibly covertly supported by England. To put down the unrest, the Duke appealed for help to Henry, who, in return, demanded the betrothal of Conan’s only daughter and heiress Constance to Henry's son Geoffrey Plantagenet.