What should one do who joined an Imam, thinking he was praying Maghrib, then it became clear to him that the Imam was praying four rak'ahs of 'Isha?

If someone prays Maghrib behind an imam who is praying four rak'ahs, and the imam stands for the fourth rak'ah, the worshiper should not follow him. Instead, he should sit for the tashahhud after the third rak'ah and either say the taslim independently or wait for the imam and say the taslim with him. The difference in the specific prayer being performed by each (the worshiper praying Maghrib and the imam praying 'Isha or another four-rak'ah prayer) does not harm the validity of the prayer. This is the madhhab of the Shafi'is, a narration from Ahmad, and the view of Tawus, 'Ata', al-Awza'i, Abu Thawr, Sulayman ibn Harb, Ibn al-Mundhir, and the Zahiris.