Must I repeat the 'Isha prayer that I performed in congregation six years ago, knowing that I was not sure whether it was three rak'ahs or four, and am I sinful for delaying knowledge of the legal ruling until now?

If someone doubts the number of rak'ahs during prayer, for instance, wondering whether they prayed three or four, they should build upon certainty, which is the lesser number (three), and then perform the fourth rak'ah. After that, they should perform the prostrations of forgetfulness (sujud al-sahw) before saying the tasleem, based on the saying of the Prophet, peace and blessings be upon him: "If one of you doubts in his prayer and does not know whether he prayed three or four [rak'ahs], let him cast aside doubt and build upon what he is certain of, then let him perform two prostrations before saying the tasleem." However, if the doubt occurs after finishing the prayer, it should be disregarded, and his prayer is valid, unless the worshipper is certain that he only prayed three rak'ahs, in which case he must repeat the prayer.