The Fourth Caliph of Islam was Ali ibn Abi Talib, who ruled from 656 to 661 CE. He was the cousin and son-in-law of the Prophet Muhammad, and is revered by both Sunni and Shia Muslims, though he holds a particularly significant position in Shia Islam as the first Imam. His leadership was marked by internal conflict and civil strife, known as the First Fitna. Ali's caliphate faced challenges, including opposition from prominent figures like Muawiya, the governor of Syria. Despite his efforts to maintain unity within the Muslim community, his rule ended with his assassination in 661 CE, leading to the establishment of the Umayyad Caliphate.