What is the ruling if a worshiper, after the first Tashahhud in the second rak'ah, recites the final Tashahhud, then remembers that he has not completed his prayer and still has the third and fourth rak'ahs remaining: Should he say the Tasleem and repeat the prayer, or should he rise for the third rak'ah?

The prayer is not invalidated by adding "Al-Salatu wa al-salamu alayka, ya Rasul Allah (Peace and blessings be upon you, O Messenger of Allah)" after the last Tashahhud and before the Taslim, even if done intentionally, because it is from the same category as the remembrances (dhikr) of the prayer. It is recommended for one who does this forgetfully to perform two prostrations of forgetfulness (sujud al-sahw).