I assume you know what sahaba means -- companions of rasulullah. The Khulafa Ar-Rashideen are four specific companions of rasulullah:
- Abu Bakr As-Siddique
- Umar ibn Al-Khattab
- Uthman ibn 'Affan
- Ali ibn Abi Talib
- Hassan ibn Ali ibn Abi Talib
To start from the beginning: rasulullah told us to follow them. Abu Dawud collects a huge hadith, which ends with:
You must then follow my sunnah and that of the rightly-guided caliphs.
Hold to it and stick fast to it. Avoid novelties, for every novelty is
an innovation, and every innovation is an error. (Abu Dawud)
This hadith establishes the status of the khulafa ar-rashideen: their sunnah is the sunnah. This comes up in issues of fiqh and other places, where their statements are taken as slightly above statements of other companions.
Another hadith mentions that the number of years of the khilafah of the khulafa ar-rashideen will be 30 years. I cannot locate this hadith presently; there are several supporting ahadith in Abu Dawud's collection, including this one (emphasis mine):
The caliphate of Prophecy will last thirty years; then Allah will give
the Kingdom to whom he wishes; or his kingdom to whom he wishes. (Abu
Also of note, one of the praiseworthy names of the righteous khalifah 'Umar ibn Abdil Azeez, the grandson of 'Umar ibn Al-Khattab, is "the fifth righteous khalifah." However, this was just a name some scholars gave him; he is not counted in the 30 years which is established from the ahadith of rasulullah.
And Allah knows best.
Source: History of the Khulafa class, circa 2006