What is the ruling on the prayer of one who led the congregational prayer for three rak'ahs in Maghrib prayer, then stood up for a fourth, and did not return after being prompted by the congregants, and did not perform the prostrations of forgetfulness (sujud al-sahw)? And is the prayer of the congregants valid in this case?

If the Imam stands for an extra rak'ah, it is prescribed for the congregants to glorify Allah (say "Subhanallah") to him. If he returns, then all is well. But if he persists, his prayer is valid unless he is certain that it is an addition. If he becomes certain and continues, his prayer becomes invalid, as does the prayer of anyone who follows him while knowing about the addition and the impermissibility of following. As for someone ignorant of this, their prayer is valid. It is recommended for both the Imam and the congregants to perform the prostration of forgetfulness (sujood as-sahw).