Fourth Buddhist Councils (29 BCE and 78 CE)
There were two of Fourth Buddhist councils held, one in Tambapanni (Sri Lanka) at Aloka Lena now Alu Vihara during the time of King Vattagamani-Abaya 29 BCE, and another in the Sarvastivada tradition, said to have been convened by the Kushan emperor Kanishka, in 78 CE at Kundalban in Kashmir.
Fourth Buddhist Council in Tambapanni (Sri Lanka)
The Fourth Buddhist Council was held in 29 BCE in Tambapanni, i.e. Sri Lanka, at Aloka Lena now Alu Vihara during the time of King Vattagamani-Abaya. By the time of the Fourth Buddhist councils, Buddhism had long since splintered into different schools.
The Dipavamsa states that during the reign of Vattagamani-Abhaya (29–17 BCE) the monks who had previously remembered the Tipitaka and its commentary orally now wrote them down in books, because of the threat posed by famine, war, and the growing power of the newly established Abhayagiri Vihara, which enjoyed the king's favour. The Mahavamsa also refers briefly to the writing down of the canon and the commentaries at this time.
The main reason for its convening was the realization that it was now not possible for the majority of monks to retain the entire Tipitaka in their memories as had been the case formerly for the Venerable Mahinda and those who followed him soon after. Therefore, as the art of writing had, by this time developed substantially it was thought expedient and necessary to have the entire body of the Buddha's teaching written down.
King Vattagamani supported the monk's idea and a council was held specifically to reduce the Tipitaka in its entirety to writing. Therefore, so that the genuine Dhamma might be lastingly preserved, the Venerable Maharakkhita and five hundred monks recited the words of the Buddha and then wrote them down on palm leaves. This remarkable project took place in a cave called, the Aloka Lena, situated in the cleft of an ancient landslip near what is now Matale. Thus, the aim of the Council was achieved and the preservation in writing of the authentic Dhamma was ensured. In the Eighteenth Century, King Vijayarajasiha had images of the Buddha created in this cave.
Fourth Buddhist Council in Kashmir
Another Fourth Buddhist Council was held in the Sarvastivada tradition, said to have been convened by the Kushan emperor Kanishka, in 78 CE at Kundalban in Kashmir. It is said that Kanishka gathered five hundred Bhikkhus in Kashmir, headed by Vasumitra, to systematize the Sarvastivadin Abhidharma texts, which were translated from earlier Prakrit vernacular languages (such as Gandhari in Kharosthi script) into the classical language of Sanskrit. It is said that during the council three hundred thousand verses and over nine million statements were compiled, a process which took twelve years to complete. Although the Sarvastivada are no longer extant as an independent school, its traditions were inherited by the Mahayana tradition.
It is said that for the Fourth Council of Kashmir, Kanishka gathered 500 monks headed by Vasumitra, partly, it seems, to compile extensive commentaries on the (Sarvastivadin Abhidharma, although it is possible that some editorial work was carried out upon the existing canon itself. The main fruit of this Council was the vast commentary known as the Mahā-Vibhāshā ("Great Exegesis"), an extensive compendium and reference work on a portion of the Sarvāstivādin Abhidharma.
Scholars believe that it was also around this time that a significant change was made in the language of the Sarvāstivādin canon, by converting an earlier Prakrit version into Sanskrit. Although this change was probably effected without significant loss of integrity to the canon, this event was of particular significance since Sanskrit was the official holy language of Brahmanism in India, and was also being used by other thinkers (regardless of their specific religious or philosophical allegiance), thus enabling a far wider audience to gain access to Buddhist ideas and practices.
For this reason, all major (Sarvastivad and Mahayana) Buddhist scholars in India thereafter wrote their commentaries and treatises in Sanskrit. Theravada however never switched to Sanskrit; the language of the Theravadin scriptures (Pali) came to be regarded as the natural language, the root language of all beings.