Praise be to Allah.

The answer to your question is a Hadith by the Prophet (May peace and blessings be upon him) in which he said:

"Allaah will build a house in Heaven for whoever is diligent in observing 12 Sunnah Rak'aat (as follows): 4 Rak'aat before and 2 after the Dhuhr (Midday) Prayer, 2 after the Maghrib (Sunset Prayer), 2 after the Ishaa (Evening) Prayer and 2 before the Fajr (Dawn) Prayer." Hadith sahih narrated by at-Tirmidhi No. 379 and by others. Hadith No. 6183 in Sahih al-Jaami.

Anbasah ibn Abi Sufyan quoted Umm Habibah as saying "Allaahs Messenger (May peace and blessings be upon him) said:

"A house will be built in Heaven for one who prays 12 Rak'aat in a day and evening as follows: 4 Rak'aat before and 2 after the Dhuhr Prayer, 2 after the Maghrib Prayer, 2 after the Ishaa Prayer and 2 before the Fajr Prayer." Narrated by at-Tirmizi under No. 380. He said: The Hadith narrated by Anbasah quoting Umm Habibah in this chapter is a hassan and sahih hadith. It is under No. 6362 in Sahih al-Jaami.

The Asr (Late-Afternoon) Prayer has no routine Sunnah (basic recommended voluntary prayer). However, it is mustahab (preferable and recommended) that one prays 4 Rak'aat before the Asr Prayer. The 4 Rak'ahs are of less reward and significance in importance of adhering to them compared to the "sunnan al-rawaatib" described above. The 4 Rak'ahs are the ones intended by the Prophet (May peace and blessings be upon him) in saying: "May Allaah have mercy on one who prays 4 rakaaat before the Asr Prayer." Narrated by at-Tirmidhi no. 395 and he declared it a hassan and ghareeb Hadith. Al-Albany rated the Hadith as hassan in Sahih al-Jami No. 3493.
All the foregoing 4-Rak'ah voluntary prayers are to be prayed two at a time according to Imam ash-Shafii and Imam Ahmad.

Allaah, the Exalted, knows best.