As the firstborn son of Diego López V de Haro, Lope Diaz IV was raised believing that he would inherit the Lordship of Biscay. After a lengthy series of disputes over the succession to Biscay which often turned violent, the title passed to María II Díaz de Haro, daughter of Maria II Diaz de Haro, Lope Diaz' cousin. With the death of Lope Díaz III de Haro who was assassinated by order of King Sancho IV of Castile in 1288, the title over Biscay passed to María II Díaz de Haro being the eldest child. After the ushering in of a new infant king Ferdinand IV of Castile after Sancho IV's death, the kingdom was thrown into chaos and Diego López V, Lope Diaz IV's father, was able to forcibly take control of Biscay away from his niece whose husband, the infante John of Castile was imprisoned at the time for crimes committed against Sancho IV in collusion with Diego Lopez III de Haro. This led to a lifelong fight between the two parties and a struggle to gain favor with the royal court, both sides having been at times considered at war with the crown. In 1307, Diego Lopez V signed an accord which granted title over Biscay to his niece, María II Díaz de Haro only upon his death and that the stolen title would remain his until such a time. The agreement also stipulated that the title of lordship over Orduña-Urduña and of Balmaseda, previously associated with Biscay, would remain in Diego Lopez V's immediate bloodline, being transferred to Lope Diaz IV upon his death. Further, in an effort to compensate and mediate this deal, the King, Ferdinand IV of Castile gave Lope Diaz IV title over Belmonte de Miranda and of :es:Villalba de Losa|Villalba de Losa, both of which had previously belonged to the crown. With regards to the accord reached over possession of Biscay, Juan Núñez II de Lara felt under compensated by the king and by his mother reason for which he withdrew from the royal courts in protest without completing his mandate. As a result, Diego Lopez V de Haro further received the title of Mayordomo mayor del Rey. This provoked further unrest as the infante John of Castile, husband of Maria II also withdrew from court in protest. By 1309, the many disputes had been resolved and all parties involved participated in a campaign against the Kingdom of Granada. This campaign, while successful in some respects such as the capture of Gibraltar, culminated in the disastrous and demoralizing Siege of Algeciras of 1309. It was here that a majority of Ferdinand IV's army would desert him, including the infante John of Castile. In 1310, Diego Lopez V died in camp of wounds received during the siege leaving his possessions with the exception of Biscay to Lope Diaz IV. In 1311, Lope Diaz IV participated in a rebellion against King Ferdinand IV of Castile together with the infante John of Castile and Juan Núñez II de Lara, head of the House of Lara. Their rebellion favored the overthrow of Ferdinand IV and the installation of his brother, the infante Pedro de Castilla y Molina instead. Eventually, the rebellion faltered after a lack of support from the queen consort, María de Molina.
Death
Lope Díaz IV de Haro died in October 1322 without leaving behind any descendants. Upon his death, all his titles passed to his brother, Fernando Díaz de Haro.
