What is the ruling on the the prayer of one who forgets the number of rak'ahs: should he base it on the lesser or the greater number?

If someone doubts the number of rak'ahs in their prayer, they should build upon the lesser number and complete their prayer, then perform the prostration of forgetfulness (sujud as-sahw), unless they are frequently afflicted by doubt, in which case they should disregard it. The evidence for this is the Prophet's (peace be upon him) hadith: "If one of you doubts in his prayer and does not know how many he has prayed – three or four – he should cast away the doubt and base it on what he is certain of, then prostrate twice before giving the salām." Ibn Qudamah said: "If he doubts while in prayer whether he has omitted a pillar of the prayer or not, his ruling is that of one who has not performed it, because the default is its absence." However, if doubts become frequent, he should disregard them and act upon what he predominantly believes to be true.