What is the ruling on praying a voluntary prayer of four rak'ahs with a single taslim?

Question:
An imam leads the people in tarawih, and he stood up for the third rak'ah, and they recited dhikr behind him, but he stood up and performed a fourth rak'ah, then said the taslim, and did not perform sujud al-sahw. He said: This is a voluntary prayer, and it does not harm you that we prayed four rak'ahs with a single taslim.
Even though he did not initially accustom the congregation to this, and did not intend it from the beginning of the prayer?

Answer:
This is the opinion of some jurists; they say: complete the fourth rak'ah, but the Sunnah is as the Prophet (peace and blessings be upon him) said: . As for if he stands up for a third rak'ah and they recite dhikr with him, he should sit and perform sujud al-sahw; this is the Sunnah. As for if he completes four rak'ahs, that is the opinion of some scholars, and it is the weaker position, but the matter is easy, if Allah wills.
Q: Okay, regarding the intention, did they start the prayer as two rak'ahs?
A: The Sunnah is that if he stands up and they alert him, he returns, or if he realizes it, he returns, and performs sujud al-sahw. He recites the tashahhud and completes the prayer, then performs sujud al-sahw. This is the relied-upon position, and it is what the hadiths indicate: . But if he completes four rak'ahs, we do not say it is invalid; there is a major disagreement among scholars on this.
Q: Even if he started with al-Fatihah?
A: Even if he started with al-Fatihah, he returns and sits, just as if he stood up for a third rak'ah in Fajr, or in Jumu'ah. Even if he started with al-Fatihah, he returns and sits if they alert him.
Q: Is his action not to be rebuked?
A: He is to be taught the Sunnah.