What should I do if I am assailed by doubt during a four-rak'ah prayer (obligatory or supererogatory) between the third and fourth rak'ahs, so I act upon the lesser number and stand up to pray an additional rak'ah with the intention of performing the prostration of forgetfulness, then while in the prayer I become certain that it has become the fifth rak'ah? Should I complete the fifth rak'ah, or should I sit down to end the prayer and perform the prostration of forgetfulness?

If a worshiper doubts during their prayer and bases their action on the minimum (number of rak'ahs), then it becomes clear to them that the rak'ah they stood up for is extra, they must immediately sit down, recite the final tashahhud, say the taslim, and perform the two prostrations of forgetfulness (sujood al-sahw). However, for one who completed the extra rak'ah out of ignorance and forgetfulness, their prayer is valid, and there is nothing upon them, as stated by Sheikh Ibn 'Uthaymeen, may Allah have mercy on him.