What should an individual worshipper do if he makes a mistake in a four-rak'ah prayer and stands up for the fifth rak'ah?

If a worshipper intentionally adds an extra rak'ah, remembering that he is adding it, his prayer becomes invalid. If the addition is due to forgetfulness (sahw), then there are two scenarios:

1. If he realizes the addition during the extra rak'ah: He must return immediately, recite the tashahhud if he hasn't already, and perform the prostrations of forgetfulness (sujud al-sahw) after the salutation.

2. If he does not realize the addition until after completing it: His prayer is valid, and he performs the prostrations of forgetfulness (sujud al-sahw) after the salutation.