He prayed a Four-Rak'ah Prayer as Three

Question :

If the Imam is in doubt during the four Rak'ah prayer and he does not know if he has prayed three or four, then he makes the Taslim, after which some of the worshippers inform him that he has only prayed three, in such circumstances, should the Imam make the openin Takbir for the fourth Rak'ah, or should he simply stand up and recite Al-Fatihah without Takbir? And when should he perform the prostration of forgetfulness, before or after the Taslim?

Answer:

If the Imam or the solitary worshipper is in doubt regarding the four-Rak'ah prayer, as to whether he has prayed three or four, he must base his reckoning on what he is sure of, which is the lesser number, and consider that he has prayed three, then make up the fourth. Then he should perform the prostration of forgetfulness before making the Taslim, based upon the authentic narration from Abu Sa'id Al- Khudri, from the Prophet, in which he said:
If any of you is in doubt in his prayer, and he does not know how many Rak'ahs he has prayed, three or four, he must cast aside doubt and base his reckoning on what he is sure of, then perform two prostrations before making the Taslim. If he has prayed five, then his prayer will be an intercessor for him; and if he has prayed correctly, then it will repel Satan.

However, if he makes the Taslim after three Rak'ahs, then he is informed of this, he should stand up without Takbir and with the intention of praying, then pray the fourth, then sit for the Tashahhud and after completing the Tashahhud, sending prayers and blessings on the Prophet and supplicating, he should make the Taslim and then perform two prostrations for forgetfulness and then make Taslim (again).

This is the best thing for every Muslim whose prayer is incomplete due to forgetfulness, based upon what has been reported from the Prophet that he made the Taslim after two Rak'ahs at Zuhr or 'Asr time, and Thul-Yadain brought it to his attention, so he stood up and completed the prayer, then made the Taslim, then performed the prostration of forgetfulness, then made the Taslim (again).

And it has been authentically reported from him that he made the Taslim after three Rak'ahs in 'Asr, and when he was made aware of this, he offered the fourth, then made the Taslim, then prostrated twice as the prostration of forgetfulness, then he made the Taslim (again).
Source:
Ibn Baz
Fatawa Islamiyah, Vol. 2 Pages 417- 418-419