Is the sitting of rest obligatory or a confirmed Sunnah when moving from the first to the second rak'ah, and from the third to the fourth in prayer?

The scholars have agreed that the sitting of rest (jalsat al-istaraha) is not among the obligations of prayer. They then differed as to whether it is a Sunnah or not. Imam Al-Shafi'i and a group of the people of Hadith (Ahl al-Hadith) hold that it is a Sunnah, adducing as proof the hadith of Malik ibn Al-Huwayrith. Whereas most scholars, such as Abu Hanifa and Malik, did not consider it a Sunnah, holding it possible that it is specific to certain situations. Others went further, stating that it is legislated when needed. The preponderant view is that it is mustahabb (recommended) absolutely, due to the legislative nature of the Prophet's (peace be upon him) action, and its establishment in the hadith of Abu Humayd Al-Sa'idi.