Is the congregant's prayer valid if the Imam added a fourth rak'ah to the Maghrib prayer and did not return after the congregants' tasbeeh, and they completed the prayer with him and performed the prostration of forgetfulness (sujood as-sahw)?

If the Imam stands for an extra rak'ah, the congregants should say "Subhanallah" to him. If he returns [to the sitting position], then so be it. If he persists, they should not follow him, because adding to the prayer while being aware [of the addition] invalidates it. The prayer of the congregants is valid if they were unaware of the ruling, and they do not need to repeat it. However, if they repeat it as a precaution, that is better. As for the Imam, his prayer is valid if he was not certain of the congregants' tasbih.