Abu Bakr was the first caliph who immediately succeeded the Prophet. He was replaced by Umar after his death.
After Umar, Uthman was selected caliph who was killed by rebels from the garrison town of Fustat in Egypt and Kufa in Iraq Ali succeeded Uthman who had to face the full brunt of the crisis of changing times when for the first time Muslims fought Muslims.
The behaviour of the four caliphs approximated to the original ideal: Abu Bakr, wise and pious; Umar courageous and just; Uthman gentle and religious; Ali brave and scholarly. The four caliphs attempted, and largely succeeded, in living up to the ideal of Islam.