Many ahadith have been related about this sunnah prayer and they all support each other.
Such hadith include the following:
Ibn ‘Umar reports that the Prophet said: “May Allah have mercy on a person who prays four rak’at before ‘asr prayer.” This was related by Ahmad, Abu Dawud, at-Tirmizhi (who calls it hasan), Ibn Hibban, and Ibn Khuzaimah. The latter two hold it as sahih. ‘Ali reports that the Prophet sallallahu alehi wasallam prayed four rak’at before ‘asr while separating every two sets of rak’at with salutations to the angels close to Allah, to the prophets, and to those who followed them – the believers and Muslims. This is related by Ahmad, an-Nasa’i, Ibn Majah, and atTirmizhi who grades it hasan.
As for praying only two rak’af at this time, this would fall under the generality of the Prophet’s statement: “Between every azhan and iqamah there is a prayer.”