The 4 Quls are known as protecting Surahs. Surah Al-Kafiroun and Al-Ikhlas protect from shirk. Surahs Al-Falaq and An-Naas protect from mischief of created things, envy, magic and the whisperings of the shaitaan.
According to the Hadith, Surah Al-Ikhlas reward is equivalent to one third of the Quran while Surah Al-Kafiroun is one quarter.
All the Surahs start with the word 'Qul' which in Arabic means 'say'. Hence the four Surahs are collectively called as 'the four quls'. Prophet Muhammad(S.A.W) would get revelations from Allah(S.W.T) which would start with the word 'say' and he would repeat exactly, the revelations to people.
The 4 Quls start as:
[109.1] Say: O unbelievers
[112.1] Say: He is Allah, the One
[113.1] Say: I seek refuge with (Allah) the Lord of Daybreak
[114.1] Say: I seek refuge with (Allah)