What is the ruling on the prayer of one who prayed over a Janazah (funeral) and the Imam said three Takbirat, forgetting the fourth Takbira?

If the Imam makes three takbirat (pronouncements of "Allahu Akbar") by mistake in the funeral prayer, the congregants must alert him. If he then returns and pronounces the fourth takbir, his prayer is valid. If he does not return, the congregants should complete the fourth takbir themselves and then perform the taslim (definition) (salutation of peace). Whoever intentionally omits a takbir has invalidated his prayer, but if he omits it by mistake, he should pronounce it as long as the interval is not prolonged. Anas (may Allah be pleased with him) once prayed and made three takbirat, then performed the taslim. When he was alerted, he faced the Qiblah, made the fourth takbir, and then performed the taslim. Sheikh Ibn Uthaymin said that if the Imam does not pay attention after being alerted, the congregant should make the takbir, and by doing so, the communal obligation (fard al-kifayah) is fulfilled.