Making up for a missed Sujud in the last Rak`ah by performing it then reciting Tashahhud and performing Sujud-ul-Sahw

A: If an Imam forgets to perform the second Sujud of the last Rak`ah and then he is reminded during the last Tashahhud, he has to perform only this one Sujud and he is to offer the last Tashahhud again then Sujud-ul-Sahw (Prostration of Forgetfulness). This is because following the prescribed sequence in Salah (Prayer) is one of its Rukns (Pillars). Regarding the case mentioned in the question, if the concerned Imam intended Sujud-ul-Sahw by the two Sujuds that he offered after being notified that he had made a mistake; his and his Ma'mums Salah is Batil (null and void). This is because the Salah lacked one of its Rukns which is the second Sujud of the fourth Rak`ah and that - as well as all the other Rukns of Salah - may not be compensated for by Sujud-ul-Sahw. Consequently, both the Imam and the Ma'mums have to make up for the whole Salah. The same is true if the Imam in question intended by the same two Sujuds to offer the two Sujuds of the last Rak`ah again without following them by the last Tashahhud as the Salah lacked another Rukn (Pillar) i.e. the last Tashahhud. This is because the Tashahhud that the concerned Imam offered before the two Sujuds in question are not counted. Since there was - in such case - a long separation between the Tashahhud and Taslim without offering Tashahhud again, both (Part No. 6; Page No. 36) the Imam and the Ma'mum have to offer the whole Salah again.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.