Is the Duha prayer, which is prayed before Dhuhr, a sound practice or a Sunnah?

The Messenger of Allah, peace and blessings be upon him, used to pray four rak'ahs after zawal (zenith) before Dhuhr. He said that it is a time when the gates of heaven are opened, and he loves that a righteous deed ascends to Allah during it. Scholars have differed regarding these rak'ahs: The Hanafis consider them to be the pre-Dhuhr sunnah (regular voluntary prayers), while the Shafi'is consider them an independent prayer called "Sunnat Az-Zawal" (the Sunnah of the Zenith). The most preponderant opinion is that they are the pre-Dhuhr sunnah, and they are performed two rak'ahs at a time.