A certain imam prayed the night prayer and concluded it after three rak'ahs, then when he had concluded, they reminded him and said, "We prayed three," so he stood up and added one, then concluded.
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The questioner : May Allah reward you, esteemed Shaykh: A certain imam prayed the night prayer and concluded it after three rak'ahs.
The Shaykh : Concluded what?
The questioner : After three rak'ahs.
The Shaykh : From three rak'ahs.
The questioner : Yes, then when he had concluded, they reminded him and said, "We prayed the three," so he stood up and added one, then concluded, then he performed the witr. Is this valid?
The Shaykh : First, may Allah bless you. It is obligatory for a person to know that the night prayer is prayed in pairs, as the Prophet (peace and blessings be upon him) said. So if he stands up for the third rak'ah by mistake, he must return when he remembers, even if he has already begun it. Indeed, Imam Ahmad (may Allah have mercy on him) explicitly stated that if a man stands up for the third rak'ah in the night prayer, it is as if he had stood up for the third rak'ah in the Fajr prayer.
Meaning: if he intentionally stands up, the prayer is invalid, but if it was by mistake, he must return for the prayer to be valid.
In the example you mentioned, we say: there is nothing wrong with him completing the four rak'ahs. He completes the four so that he does not have two witr prayers, then when he has completed it, he prays the witr as this man did, and there is nothing wrong with that.
The Shaykh : Concluded what?
The questioner : After three rak'ahs.
The Shaykh : From three rak'ahs.
The questioner : Yes, then when he had concluded, they reminded him and said, "We prayed the three," so he stood up and added one, then concluded, then he performed the witr. Is this valid?
The Shaykh : First, may Allah bless you. It is obligatory for a person to know that the night prayer is prayed in pairs, as the Prophet (peace and blessings be upon him) said. So if he stands up for the third rak'ah by mistake, he must return when he remembers, even if he has already begun it. Indeed, Imam Ahmad (may Allah have mercy on him) explicitly stated that if a man stands up for the third rak'ah in the night prayer, it is as if he had stood up for the third rak'ah in the Fajr prayer.
Meaning: if he intentionally stands up, the prayer is invalid, but if it was by mistake, he must return for the prayer to be valid.
In the example you mentioned, we say: there is nothing wrong with him completing the four rak'ahs. He completes the four so that he does not have two witr prayers, then when he has completed it, he prays the witr as this man did, and there is nothing wrong with that.