Is the prayer of the congregant valid if he completes two rak'ahs individually after the imam, who erred in his prayer, has said the taslim?

If the Imam and the congregants perform a fourth raka'ah, their prayer is valid. Following the Imam in sitting for the first tashahhud out of forgetfulness or ignorance does not invalidate the prayer. What was obligatory upon the congregant was to follow the Imam until he completed his prayer, and then the congregant would make up the missed raka'ah. There is no harm in the addition due to forgetfulness or ignorance, and there is no prostration of forgetfulness (sujood as-sahw) required of the congregant, because the Imam bears his forgetfulness. Separating from the Imam unintentionally does not invalidate the prayer.