Was the Friday Prayer (Salat al-Jumu'ah) originally four rak'ahs and then legislated as two rak'ahs? If so, what was the reason for that, and when did it occur?

There is no evidence that the Friday prayer (Salat al-Jumu'ah) was initially four rak'ahs and then converted to two; rather, it appears that it was legislated as two rak'ahs only. This is indicated by the saying of Umar ibn al-Khattab, may Allah be pleased with him: "The prayer of travel is two rak'ahs, the prayer of Eid al-Adha is two rak'ahs, the prayer of Eid al-Fitr is two rak'ahs, and the Friday prayer is two rak'ahs—complete, not shortened, as spoken by Muhammad, may Allah's peace and blessings be upon him."