What should be done if extra or fewer takbirs are recited during the funeral prayer?

The funeral prayer is a prayer without bowing (ruku) and prostration (sujud); its pillars are standing (qiyam) and the takbirs. There are four takbirs in the funeral prayer, including the opening takbir (takbir al-ihram). Saying the salam at the end is obligatory (wajib). Its sunnahs are to praise and glorify Allah, to send blessings and peace upon the Messenger of Allah (s.a.s.), and to pray for both the deceased and the Muslims. If the imam recites more than four takbirs, the congregation does not follow him in this; they wait for him to give the salam so that they can finish the prayer with him. However, if the imam forgetfully recites fewer takbirs and gives the salam, the prayer of the imam becomes invalid, and even if the congregation completes the fourth takbir on their own, the prayer is not valid. In this case, the prayer must be performed again. If it is realized after the burial that the prayer was performed with fewer takbirs, and not much time has passed since the burial, the prayer is performed over the grave (Kâsânî, Bedâi‘, 1/313-315).

Source: T.C. Diyanet İşleri Başkanlığı, Din İşleri Yüksek Kurulu