What is the virtue of praying four rak'ahs before the 'Asr prayer, and is it permissible to perform them after the Imam has called the iqama for prayer?

Praying four rak'ahs before Asr is a well-established Sunnah from the Prophet, peace and blessings be upon him. It is a supererogatory prayer (nafilah), and its time extends from the beginning of the Asr prayer time until the commencement of the congregational Asr prayer. If the prayer has been commenced, then supererogatory prayer is not permissible, based on the Prophet's saying, "When the prayer is commenced, there is no prayer except the obligatory one."