When one prays the four Sunnah rak’ahs of Zuhr, before or after [the fard prayer], or the Sunnah of ‘Asr, should he say the Salaam once or twice? [i.e., should he put all four rak’ahs together, or pray them two by two?]
Praise be to Allaah.
It is permissible for him to pray them with one tashahhud or two [i.e., he may pray them all together or two by two], but it is preferable to say the salaam twice [to pray them two by two].
From Fataawa al-Imaam al-Nawawi, p. 51.