Should one who intends to pray the Sunnah pray four rak'ahs or two if he prays with a congregation performing the 'Ishā' prayer?

Firstly: It is preferable for the regular Sunnah prayers (Sunan Rawatib) not to be performed in congregation, as this was the guidance of the Prophet (peace be upon him). However, it is permissible to pray them in congregation occasionally, and there is no harm or dislike in doing so. Some supererogatory prayers are legislated to be performed in congregation, such as the prayer for rain (Salat al-Istisqa), the eclipse prayer (Salat al-Kusuf), and the Tarawih prayer. Other supererogatory prayers, however, are not Sunnah to be performed in congregation, such as the regular Sunnah prayers accompanying the obligatory prayers and the night prayer outside of Ramadan. Nevertheless, there is no harm in performing them in congregation occasionally.

Secondly: If a person wishes to join a congregation performing the Isha prayer with four rak'ahs, while intending the post-Isha regular Sunnah prayer, there are four options available to him:

1. The Best Option: To pray Isha again with them with the intention of a supererogatory prayer. The evidence for this is the hadith of Abu Sa'id al-Khudri that the Prophet (peace be upon him) said: "Who will give charity to this one by praying with him?!" Performing a second congregational prayer with the intention of a supererogatory prayer is permissible, even if it is during a prohibited time, because it has a specific reason. 2. The Permissible Option: To pray the four rak'ahs with them with the intention of the night prayer (Qiyam al-Layl). It is reported from Aisha (may Allah be pleased with her) a description of the Prophet's (peace be upon him) night prayer as four connected rak'ahs. 3. The Permissible Option: To pray two rak'ahs with the intention of the Isha regular Sunnah prayer, then wait for the Imam until he sits for the tashahhud and salam, and then make the salam with him. 4. The Permissible Option: To pray two rak'ahs with the intention of the Isha regular Sunnah prayer, then intend to separate from the congregation and make the salam before the Imam's salam. This is permissible due to a legitimate Shari'ah or sensory excuse.

The first option is the best, followed by the second. As for the third and fourth options, they should not be chosen due to their deviation from the manner of the Imam's prayer.