If the Imam is in Doubt as to whether He has prayed Three or Four Rak'ahs in the Four-Rak'ah Prayer

Question :

If the Imam is in doubt during the four-Rak'ah prayer, and he doesn't know whether he has prayed three or four, then he makes the Taslim and after the Taslim, some of the worshippers informed him that he had only prayed three- in this situation, should the Imam make the opening Takhir for a four-Rak'ah prayer, or should he just stand and recite Al-Fatihah without Takbir... and when should he perform the prostration of forgetfulness, before the Taslim, or after it? I implore you to inform us, and may Allah make you successful.

Answer:

If the Imam or the single worshipper is in doubt in the four-Rak'ah prayer over whether he has prayed three or four Rak'ahs, it is incumbent upon him to assume the lesser number in order to be sure, so he should consider it to be three and then pray the fourth, then perform the prostration of forgetfulness before making the Taslim, because it has been authentically reported from the Prophet that he said:
"If any of you is in doubt regarding his prayer, and he does not know how much he has prayed, three or four (Rak'ahs), then let him cast aside doubt and base his estimation upon what he is sure of. Then prostrate twice before making the Taslim; if he has prayed five, his prayer will be an intercessor for him, and if he has completed the prayer correctly, it will be a humiliation for Satan.

However if he has made the Taslim after three, then he was informed of this, he should stand up without Takbir, with the intention of praying, then perform the fourth Rak'ah, then sit for the Tashahhud. After finishing the Tashahhud and sending prayers on the Prophet and making supplications, he should say the Taslim, then perform the prostration of forgetfulness, then make the Taslim (again). It has been authentically reported from him that he made the Taslim after praying three Rak'ahs in the Zuhr or the 'Asr prayer, and Thul-Yadain informed him of this and so he stood up and completed his prayer, then made the Taslim, then made the prostration of forgetfulness, then made the Taslim (again).

And it has been authentically reported from him that he made the Taslim after praying three Rak'ahs in the 'Asr prayer, then when he was informed of that, he prayed the fourth, then made the Taslim, then prostrated twice due to forgetfulness, then he made the Taslim (again).

Source:
Ibn Baz
Fatawa Islamiyah, Vol. 2 Pages 268-269