Q: Should I make voluntary prayers in sets of two rakahs or can I make four together?
A: It is preferred to pray in sets of two for voluntary prayers made at night and during the day.
Praying four together during the day is permissible and if done it is preferred to have both tashahhuds.
An additional surah should be recited after al-Fatihah in each rakah, even if it exceeds four during the day or two at night.
If more than eight are prayed with a single taslim, it is valid though disliked. In the case of witr and duha prayers, it is not disliked as both have been narrated.
It is also valid, though disliked, to pray an odd number like one, three, or five.
Source: Hidayat al-Raghib