Sunnahs. 4 rakats or 2?

The Virtues Of The Sunnah Of Zuhr Salah

Umm Habibah (radiallahu anha) reported: The Messenger of Allah (salallahu alayhi wa salam) said, "Whoever observes the practice of performing four Rak`ah before Zuhr prayer and four (2 sunnah, 2 nafl) after the Zuhr prayer, Allah will shield him against the Fire (of Hell)."
[Abu Dawud and At-Tirmidhi].

This Ahadith mean that a person who follows this course of practice, will die as a Muslim and will not remain in Hell for ever like the Kuffar (infidels) unless Allah has forgiven all his sins for him and would, as a result, save him from Fire altogether. That is, Almighty Allah will not let him live in Hell for ever. According to some Ahadith, the fire of Hell will not touch him, which also amounts to saying that he will not be kept in Hell for all eternity. If a Muslim is liable to punishment, his stay in Hell - for a few days or weeks or months depending on the nature of his sins - is not a contravention of such Ahadith because he will ultimately be released from Hell and brought to Jannah. "Allah will shield him against the Fire'' should not be taken to mean that a Muslim will not be sent to Hell no matter what he does. If Almighty Allah does not forgive him in the very first instance, he will have to suffer the torment of Hell as long as He would like and then he will be sent to Jannah.

`Abdullah bin As-Sa'ib (radiallahu anh) reported: The Messenger of Allah (salallahu alayhi wa salam) used to perform four Rak`ah prayer after the declining of the sun before Zuhr prayer and would say, "This is an hour at which the gates of heaven are opened, and I like that my good deeds should rise to heaven at that time.''
[At-Tirmidhi]

The Prophet (salallahu alayhi wasalam) used to perform the four Rak`ah Sunnah of Zuhr prayer when the sun started declining. In fact, except for `Isha' prayer, he would perform every Salat at its early hours.

The phrase "the gates of heaven are opened'' means that the good deeds that people do are lifted to the heavens at that time.

`Aishah (radiallahu anha) reported: If the Prophet (salallahu alayhi wa salam) could not perform four Rak`ah before Zuhr prayer, he would perform them after it (i.e., after the obligatory prayer).
[At-Tirmidhi]

This Hadith tells us about the preparation which the Prophet (salallahu alayhi wa salam) used to make for performing the Sunnah. Every Muslim should, therefore, make full preparation for performing Sunnah. If one is unable to perform it before the Fard prayer, then one must do it afterwards.

The Virtues Of The Sunnah Of Asr Salah

Ibn `Umar (radiallahu anhuma) reported: The Prophet (salallahu alayhi wasalam) said, "May Allah have mercy on a man who performs four Rak`ah before the `Asr prayer.''
[Abu Dawud and At-Tirmidhi]

The `Ulama' have stated that these four Sunnah of `Asr prayer are Ghair Mu'akkadah (non-emphasised but praiseworthy to perform). Its importance is however evident from the fact that the Prophet (salallahu alayhi wa salam) prayed for mercy on those who performed these Sunnah.

.... For those that are non-emphasized sunnas, one follows the tashahhud in the first sitting with sending blessings upon the Prophet before standing for the third rak`a, which one then begins with the opening supplication (thana`) and seeking refuge (ta`awwudh) before the basmala and Fatiha.

Source: Maraqi 'l-Sa`aadat, Page 98, footnote #339