Historical Answer
The first caliph was Abu Bakr who ruled from June 632 C.E. to August 634 C.E.
Shiite Answer
The Prophet himself declared, in front of many people returning home from Hadj, that Ali ibn Abi Talib ( A. S. ) Was to be the Imam ( Leader ) after him. However, Abu Bakr got himself elected by a Shoura in contravention to the Prophet's Will. As a result, he was a false Caliph. Ali was finally recognized as Caliph by all Muslims in 656 C.E.
First 4 Caliphs were not missionaries to Africa , but many other companions of prophet were .
The Umayyad Caliphate came after the four Rightly-Guided Caliphs.
they ruled differently
None. Caliphs came to power in the Medieval Period. As for the Medieval period, the Caliphs were the leaders of the Muslim Empires called Caliphates. The four first Caliphs were called the Rightly-Guided Caliphs and they also had some religious authority in Islam. Because of the barbarity of the Umayyad Caliphs, the religious authority left the Caliphs and vested in the local Imams. Religious authority would return to the Caliphs in the mid-1500s in the Ottoman Empire until Atatürk abolished the Caliphate in 1936.
All the first Four Caliphs were chosen in different ways. Later on it became a kind of Kingship, son following the father.
they ruled differently
to northwest europe
they were the damascus
The first Four Caliphs are Called the Rightful (Rashidoon) Caliphs. They were in order of seniority: Hazrat Abubakr, Hazrat Umar, Hazrat Usman and Hazrat Ali (May Allah be pleased with them). The fifth Caliph was Hazrat Hasan (RA) who surrenderd his right to Hazrat Amir Muawya (RA)
Sunni
Iraq
The first Four Caliphs are Called the Rightful (Rashidoon) Caliphs. They were in order of seniority: Hazrat Abubakr who died his natural death. Hazrat Umar was assasinated in the Majid i - Nabwi when he came to lead the Fajr prayer. Hazrat Usman was assasinated by the rebels. Hazrat Ali was also assinated while he came to lead the Fajr prayer. In fact, the first four Caliphs didn't keep any body guard.