How many Rakats are there in the Sunnah prayer for Dhuhr?

The Sunnah of Dhuhr prayer is two rak'ahs before it and two rak'ahs after it. It has also been reported that one prays four rak'ahs before it and two rak'ahs after it.

Ibn 'Umar, may Allah be pleased with him, memorized ten rak'ahs from the Prophet, peace and blessings of Allah be upon him, among which were two rak'ahs before Dhuhr and two rak'ahs after it.

The Prophet, peace and blessings of Allah be upon him, never used to abandon four rak'ahs before Dhuhr. This does not contradict the Hadith of Ibn 'Umar, because it is an addition that 'Aisha, may Allah be pleased with her, knew, but Ibn 'Umar did not.