Alan IV (born circa 1063; died 13 October 1119) was Duke of Brittany from 1084 until his abdication in 1112. He was also Count of Nantes and Count of Rennes. His parents were Hawise, Duchess of Brittany and Hoel II. He is also known as Alan Fergant. ("Fer gant" in French means "Iron glove".) Through his father, he was of the Breton House of Cornouaille dynasty (Breton: Kerne dynasty). He was the last Duke of Brittany to speak the Breton language.

A traditional rivalry between Brittany and Normandy continued at the close of the 11th century. The Breton-Norman war of 1064?1065 was the result of William the Bastard, Duke of Normandy (later to become known as William the Conqueror) support of rebels in Brittany against Alan's maternal uncle, Conan II.

Conan II died in late 1066 during a campaign in Anjou, and was succeeded by Alan's parents, Conan's sister Hawise as Duchess and her husband Hoel II as Consort. Hoel ruled as Regent of Brittany from Duchess Hawise's death in 1072 until Alan reached his majority in 1084.

Arthur Collins described either Alan IV or Alan Rufus (the two of whom he confuses) as follows: "In his youth he was not a little famous for his valour, insomuch as he feared not William, then Duke of Normandy". To prevent further hostilities during his invasion of England, William I married his daughter Constance to the new duke Alan in 1087. The marriage ceremonies may have taken place in Bayeux in Normandy. William of Malmesbury wrote that Constance was unpopular at the Breton court because of her "severe and conservative" manner. William of Malmesbury also alleged that Alan IV had Constance poisoned to death, but this remained unverified However, Orderic Vitalis wrote that as duchess Constance did all she could to further the welfare of the Bretons, who grieved deeply at her death in 1090.

In 1092 Alan IV donated property to the abbey of Redon by charter, and by 1093 married Ermengarde of Anjou, as a political alliance with Fulk IV of Anjou to counter Anglo-Norman influence.

Duke Alan IV's cousin Geoffrey I Boterel (eldest brother of Alan Rufus) died on 24 August 1093 in battle at Dol while in rebellion against the Duke.

Alan IV abdicated as Duke in 1112. The former Duke retired to the monastery of Redon, where he died in 1119.
In 1098 Alan went on the First Crusade, leaving Ermengarde as his regent, and returned in 1101 (wikipedia)