What should one do who doubts during Dhuhr prayer whether he is in the fourth or fifth rak'ah, and how does he distinguish between illusory and real doubt?

Whoever doubts in his prayer whether he prayed four or five [rak'ahs], he should consider it four, and no prostration of forgetfulness (sujud sahf) is required of him; because the default is the absence of addition. Doubt is the equality of two possibilities, while conjecture (zann) is the preponderance of the correct side, and delusion (wahm) is the preponderance of the incorrect side. No prostration of forgetfulness (sujud sahf) results from delusion (wahm).