What is the ruling on someone who prayed the four rak'ahs of 'Isha', then sat during the third rak'ah and said the taslim, then stood up and repeated his prayer after being alerted by the worshippers, without having moved from his place?

The Imam was obligated to make up the forgotten رک'ah and perform the prostrations of forgetfulness (sujud al-sahw). His speech, while believing his prayer was complete, does not invalidate it. As for the congregants who spoke to him intentionally to inform him, their prayer is invalidated according to the majority of scholars, except for the Malikis, who hold that slight speech for the benefit of the prayer does not invalidate it. They cite the hadith of Dhul-Yadain as evidence. The Imam should have completed his prayer and not قطع it, and those congregants who spoke to him should have started a new prayer according to the majority. All of them are absolved of blame by repeating the prayer.