How is the supererogatory prayer (nafilah) before Dhuhr and before Asr performed: Is it four rak'ahs with one taslim or two rak'ahs with a taslim for each two?

The majority of scholars are of the view that it is preferable for supererogatory prayers (nafl) during the day and night to be performed two rak'ahs by two rak'ahs, based on the Prophet's (peace be upon him) saying: "The prayer of the night and day is two by two." Some scholars, such as Imam Ahmad, held this to be obligatory, and that the prayer is invalid if performed more than two rak'ahs with a single tasleem, except for the Witr prayer, as authentic Sunnah has been reported regarding it. Reports of prayers of four rak'ahs are understood to mean they were performed with two tasleems.