How are four rak'ahs performed before the Friday Dhuhr prayer, and is it permissible to perform them before the decline of the sun from its zenith (Zawal)?

There is no regular (ratibah) Sunnah prayer before the Friday prayer. This has not been authentically reported from the Prophet (peace be upon him) nor from his companions, and it is not permissible to analogize it to the Dhuhr prayer. As for what has been reported from Ibn Mas'ud and other companions, it was a voluntary act on their part. What is prescribed for a Muslim is to pray as much voluntary (nafl) prayer as he can from the moment he enters the mosque until the Imam comes out for the prayer.