Is there a sin upon us or upon the Imam for not performing the prostration of forgetfulness (sujud al-sahw) after he stood up for the fourth raka'ah, having forgotten to sit for the second tashahhud in Dhuhr prayer? And is it permissible for him to prostrate after the taslim alone, or should we prostrate with him?

Prostration of forgetfulness (sujood as-sahw) is obligatory for the Imam according to the Hanbalis, and sunnah according to the majority of scholars, and its obligation is more likely here. Scholars differed regarding the place of sujood as-sahw, and the more likely opinion is that the Imam performs it after the salaam for an addition, which is the view of the Malikis. The prescribed act for the Imam was to prostrate, and it is better to do so after the salaam, and the congregants (ma'moomeen) should follow him. The prayer is valid for not performing the prostration out of ignorance or forgetfulness according to the Hanbalis. However, the congregant should have performed sujood as-sahw if the Imam omitted it. Otherwise, the prayer is valid in any case, Allah willing.