Is what the questioner did—making up the Witr prayer as four rak'ahs with two tashahhuds and without qunut—correct, and must he make up the Witr prayer if he was ignorant of having missed the prayer for many years, and if the answer is yes, is it permissible to make it up as only one rak'ah?

The majority of scholars hold that Witr prayer is Sunnah (recommended) and not Wajib (obligatory). The strongest evidence for this is the Hadith narrated by Talha ibn Ubaydillah (may Allah be pleased with him), in which the Messenger of Allah (peace and blessings be upon him) said: "Five prayers in a day and a night." [A man] asked: "Is there anything else obligatory upon me?" He said: "No, unless you perform voluntary prayers."

Therefore, the questioner is not obligated to make up a missed Witr prayer, but it is recommended for him to do so. It is permissible to perform Witr with a single Rak'ah, as stated in the Hadith: "And if you fear the dawn, then pray Witr with one [Rak'ah]." And what he did by making up the Witr as an even number of Rak'ahs with two Tashahhuds is correct.