Question: Is there anything wrong with performing post-fard sunnat salats of Salat az-Zuhr and Salat al-Isha as four rak’ats?
ANSWER
There is nothing wrong with it, and it is better to do so.
The salats other than fard and wajib ones are called voluntary (nafilah) salats. Sunnat salats fall into this category, too. It is permissible to offer voluntary salats anytime except the times during which it is makruh to perform salat.

The following are some hadith-i sharifs about the virtue and significance of offering a four-rak’at voluntary salat after the fards of Salat az-Zuhr and Salat al-Isha and a six-rak’at voluntary salat after the fard of Salat al-Maghrib:

(Hell will be haram for a person who is constant in performing a four-rak’at salat before and after Salat az-Zuhr.) [Tirmidhi]

(Whoever performs six rak’ats after Salat al-Maghrib, his sins will be forgiven, even if they are as abundant as the foam of the sea.) [Tabarani]

(If a person offers six rak’ats after Salat al-Maghrib without saying anything in between, he will attain rewards as if he had done voluntary acts of worship for twelve years.) [Ibn Majah]

(A four-rak’at salat offered before the fard of Salat az-Zuhr is similar to a four-rak’at salat offered after the fard of Salat al-Isha. A four-rak’at salat offered after Salat al-Isha equals in reward a four-rak’at salat offered on the Night of Qadr.) [Tabarani]

However, note that to be able to attain rewards in return for voluntary acts of worship one must not have a fard obligation (qada) to make up. One cannot earn any rewards in return for one’s voluntary acts of worship that are of the same type unless one makes it up. A hadith-i sharif written in the book Durrat-ul-fakhira states, “Allahu ta’ala will not accept the voluntary salat of a person who has a salat to make up [qada salat].” Secondly, to attain rewards in return for voluntary acts of worship, one must abstain from haram acts and repent of one’s sins. (Nawadir-i fiqhiyya)