What is the ruling on the prayer led by an Imam who prayed three rak'ahs for 'Isha, then added a fourth rak'ah and performed the prostration of forgetfulness (sujud al-sahw), knowing that some of the congregants joined late?

If someone prays three rak'ahs for Isha by mistake and then says the taslim, he should stand up to complete his prayer, then say the taslim, then perform the prostrations of forgetfulness (sujud al-sahw), and then say the taslim again, as the Prophet (peace be upon him) did.

And if the Imam says the taslim due to an incomplete prayer, and the latecomer (masbuq) stands up to make up what he missed, then it becomes clear that the Imam still has one rak'ah remaining and stands to perform it, then the latecomer has a choice: if he wishes, he can return and pray that rak'ah with his Imam, then stand up and complete what he missed of the prayer; and if he wishes, he can continue his prayer individually.