Rashidun

Abu Bakr 632-634
Umar ibn al-Khattab 634-644
Uthman ibn Affan 644-656
Ali ibn Abu Talib 656-661

Usually called al-khulafa' al-Rashidun in Arabic, the Rightly Guided Caliphs were so called because no Caliph after them was perceived as an undisputed leader by all Muslims. Every ruler of the Islamic community after Ali ibn Abu Talib has had his right to rule challenged by various opposition groups.

Moreover, after Ali's death and the ascension of the Umayyads, the succession in the Caliphate turned hereditary, something which was profoundly against the old Arab way of choosing a leader ('sheikh') amongst the most able members of the tribe. All the four Caliphs before Mu'awiyya had been elected by the Muslim community, whereas Mu'awiyya managed to seize power by force and established a precedent of dynastic succession.

It is also true that the first four Caliphs had been close companions of the prophet Muhammad and thus had the benefit of having heard his teachings with their own ears. Two of them, Abu Bakr and Ali, had been among the first converts when Muhammad began his mission of spreading Islam. They had no need for Quranic interpretations. In the early days of Islam, people simply asked the prophet himself to explain difficult passages. Latter Caliphs lacked this personal guidance and were perceived as more erratic in their interpretation of the Quran and its laws.

The first Caliphs were said to have lived pure, simple lives without the royal pomp and splendour later favoured by the Umayyads and the Abbasids. They did not perceive themselves as kings but spiritual guardians of the Umma, the Muslim community. Unfortunately, during the centuries that followed, the Caliphs lost this 'first among equals' image of themselves, and the Caliphate deteriorated rapidly into a mere temporal kingship.

http://www.usc.edu/dept/MSA/politics/firstfourcaliphs.html
Encyclopedia of Islam
http://islamicweb.com/history/bio_caliphs.htm