Why did the Prophet (peace be upon him) specifically mention four individuals from whom to take the Qur'an?

Al-Bukhari and Muslim reported on the authority of Abdullah ibn Amr that the Prophet said: "Learn the Qur'an from four: from Ibn Mas'ud, Ubayy ibn Ka'b, Mu'adh ibn Jabal, and Salim, the freed slave of Abu Hudhayfah." Scholars explained this by saying that they were more precise in its wording and more perfect in its performance, or that they dedicated themselves to receiving it orally from the Prophet, or that they dedicated themselves to having it taken from them, or that this was an announcement of their advancement and mastery of the Qur'an after the Prophet's death.