Dought & clear, - His imam said three takbeers in the funeral prayer, and forgot the fourth; should he followhim?

Some people offered the funeral prayer, and the imam said three
takbeers, but did not say the fourth because he forgot. What is the
ruling on his prayer?
Praise be to Allah.
In the answer to question no. 159418we stated that the number of
takbeers in the funeral prayer is four, allof which are pillars or
essential parts of the prayer.
Based on that, if the imam says only three takbeers by mistake, he
should not be followed, and it is essential to alerthim to that. If he
then says the fourth takbeer, his prayer is valid, but if he does not
do that, then the people praying behind him have to complete the
fourth takbeer, then say the salaam.
Al-Bahooti (may Allah have mercy on him) said:If a worshipper omits
one of the four takbeers deliberately, his prayer becomes invalid,
because he has omitted an obligatory part deliberately; hence it
becomes invalid, as in the case of all other prayers. If he omitted it
by mistake, he should say it; this is similar to the case if he says
the salaam in an obligatory prayer before completing it by mistake– he
should (correct his mistake) if it is soon afterwards, and his prayer
is still valid.
End quote from Daqaa'iqOoli an-Nuha, 1/362
Al-Bukhaari narrated, in the chapter on saying takbeer in the funeral
prayer: Humayd said: Anas (may Allah be pleased with him) led us in
offering the (funeral) prayer, and he said takbeer three times, then
he said the salaam. It was pointed out to him, so he turned to facethe
qiblah, then he said the fourth takbeer, then he said the salaam.
Al-Haafiz (may Allah havemercy on him) said: 'Abd ar-Razzaaq narrated
from Ma'mar, from Qataadah, from Anas, that he said takbeer three
times in the funeral prayer, then he ended it by mistake. They said: O
Abu Hamzah, you only said takbeer three times. He said: Get back into
your rows. So they did so, andhe said the fourth takbeer.
End quote fromFath al-Baari, 3/202
Shaykh Ibn 'Uthaymeen (may Allah have mercy on him) was asked: If
theimam says the takbeer three times (in the funeral prayer) then
saysthe salaam, what should the one who is praying behind him do? He
replied: He has to alert him to that.
Question: What if he is not alerted to it?
Shaykh: If he is not alerted to it, then the one who is praying behind
him should say the takbeer, and thus the communal obligation will have
been fulfilled.
End quote fromash-Sharh al-Kaafi.
And Allah knows best.