Does the traveler complete four rak'ahs, or does he prostrate for forgetfulness if he adds a rak'ah out of forgetfulness?

Question:
Honorable Shaykh, may Allah grant you success. This questioner says: If a traveler enters the prayer with the intention to shorten it, but adds a rak'ah out of forgetfulness, does he complete it as four, or does he prostrate for forgetfulness?

Answer:
He prostrates for forgetfulness and does not complete it as four. He prostrates for forgetfulness. Yes.