All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
In principle supererogatory prayers should be performed in twos. The Prophet said: "The prayer (voluntary prayer) of the night and day is in twos." [Ibn Maajah, At-Tirmithi, An-Nasaa'ee and Abu Daawood]. Nevertheless, it is permissible for a person to perform four Rak'as with one Salaam.
'Aasim Ibn Dhamrah, said: 'I asked 'Ali Ibn Abu Taalib about the supererogatory prayers of the Prophet during the day before the obligatory ones, and he replied: "Who can do what he did?" And then he informed us saying: ''The Prophet used to perform two Raka'hs when the sun reaches its meridian, and four Raka'hs before midday, with one salaam at the end.' [An-Nasaa'ee, and Al-Albaani classified it as authentic]
The scholars stated that the narration reported by Ibn 'Umar is about the superiority [that it is better to pray in twos], and the narration reported by 'Ali is about the permissibility [of doing i.e. performing four Raka'hs with one Salaam].
Allaah Knows best.