The four caliphs were:
Abu Bakr (ra) - best friend of Prophet Muhammad (saw). He oversaw the integration of the Arab Peninsula and sent cavalry to fight the hooligans on the fringes of the peninsula declaring prophethood for themselves for political luxury.
Umar ibn al Khattab (ra) - 2nd most close companion of Prophet Muhammad (saw). He sent an army under some Companions to fight against Sassanid Persian Empire, effectively destroying it. He also conquered Byzantine Egypt and Lybia. He was killed by a Zoroastrian.
Usman ibn Affan (ra) - third in line of the Khilafat; he was the Prophet's son in law. He lived during a time of expansion and immense luxury, but he also lived at a time of political corruption. He was killed because some elements of society blamed him for the corruption of their governor-generals.
Ali (ra) - he is the last of the four and the reason of the Sunni-Shia political split. His time marked more expansions but at the same time the political incohesiveness due to the death of Uthman ibn Affan. Certain elements of society wanted him to take justice, others wanted him to take another path. You really have to read into it to understand the political turmoil. Anywho, he was assasinated by rebellious elements. His children were murdered at the hands of the Ummayad army.
These four are considered to have reigned over the Muslim Ummah at its peak. In the end it steadily got worse until Ali's (ra) death.
Ever since then, Muslim Scholars consider the Ummah, or collective body, as in a state of impeding degeneration and decadence.