Assalaamu alaykum wa rahmatullaah. Please, if I joined a congregation to observe the Magrib prayer after the congregation prayed one unit of prayer while it was unknown to me dat they were praying the ‘Ishaa’, which is four units of prayer, what happens to my prayers?

All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallaahu ʻalayhi wa sallam, is His slave and Messenger.

If you joined the congregational prayer with the intention of performing the Maghrib while the congregation were in the second Rakʻah (unit) of the ʻIshaa’ prayer, your Maghrib prayer with them is valid according to the preponderant view held by some scholars in this regard. The latecomer, in this situation, should follow the imaam whenever he sits or stands throughout the prayer.

An-Nawawi wrote, “If the number of Rakʻahs performed by the latecomer is less than the imaam's, such as praying the Fajr behind an imaam performing a four-Rakʻah prayer or the Maghrib, or praying the Maghrib behind an imaam performing a four-Rak‘ah prayer, there are two views in this regard that were narrated by the scholars of Khuraasaan; the more correct of these two views, which was also held as decisive by the scholars of ʻIraq, is that his prayer is valid as in the opposite case (i.e. when the number of Rakʻahs performed by the latecomer is more than the imaam's). The second view narrated by the scholars of Khuraasaan is that there are two opinions regarding it, the most correct of which is this (aforementioned) opinion and the second of which is that it is invalid.” [Al-Majmoo’]

Ibn ʻUthaymeen said:

“If the latecomer joins the congregational prayer with the intention of performing the Maghrib while the imaam is in the second Rakʻah of the ʻIshaa’, he should make Tasleem (saying: Assalaamu 'alaykum) at the end of the prayer with the imaam, and there is no harm in that. Some people say: ‘If he joined the imaam in the second Rakʻah, he would sit in his first Rakʻah because his first Rakʻah is the imaam's second Rakʻah. The same happens when the imaam stands up to perform the fourth Rakʻah; the latecomer is supposed to sit down because he is performing the Maghrib and needs to sit down to recite the Tashahhud. However, he does not sit but rather follows the imaam, so he recites Tashahhud in other than its place and leaves it in its place.’ We say: this difference has no effect (on the validity of the prayer). Do you not see: if he joins the imaam in the Thuhr prayer in the second Rakʻah (with the intention of performing the Thuhr), will the manner of performing the prayer differ or not? Yes, it would be different, because if he joins the Thuhr prayer with the imaam in the second Rakʻah with the intention of performing the Thuhr, then he will recite the first Tashahhud in his first Rakʻah and leave Tashahhud in his second Rakʻah and then recite Tashahhud again in his third Rakʻah. There is no harm in that, because what he did in following the imaam does not affect the validity of his prayer because he is ordered to follow the imam.” [Al-Liqaa' Ash-Shahri]

Hence, your Maghrib prayer behind the imaam who was performing the ʻIshaa’ is valid, Allah willing.

Allah knows best.