Is it permissible to perform supererogatory prayers as four rak'ahs or two by two?

It is recommended to make up missed timed supererogatory prayers, such as the regular sunnah prayers (rawātib), Duha prayer, and the Eid prayers. This is the view of the Shafi'i school and the well-known opinion among the Hanbalis. However, non-timed supererogatory prayers, such as the eclipse prayer (salat al-kusuf), the prayer for rain (salat al-istisqa'), and the prayer for greeting the mosque (tahiyyat al-masjid), are not to be made up. It is also recommended to make up the regular supererogatory prayers in their original form. Thus, it is permissible to make up the pre-Dhuhr and post-Dhuhr sunnah prayers as four rak'ahs with one salām, and the Witr prayer as three or five rak'ahs with one salām.