What is the ruling on the prayer of someone who, out of forgetfulness, added a fourth rak'ah to Maghrib prayer, then remembered while standing, so he sat and completed his prayer?

If someone stands up for an extra rak'ah after the last rak'ah and then remembers, they must sit down and perform two prostrations of forgetfulness (sujud al-sahw) before the salutation (taslim). If they omit the sujud al-sahw in this situation due to forgetfulness or ignorance, their prayer is valid and they are not required to repeat it.