It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that in the last funeral prayer that he (peace and blessings be upon him) offered, he said four takbirs.
Al-Nadr bin Abd al-Rahman Abu Umar al-Khazzaz is alone in narrating it from Ikrimah, and he is da'if (weak). This wording has been narrated through other routes as well, all of them weak; however, the agreement of most of the Companions (may Allah be pleased with them) on four is like evidence for it, and Allah knows best.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنائز / 6948