The Fourth Council
There are two separate councils that are commonly referred to as the Fourth Council, which occurred nearly 200 years apart from each other. The first occurred in Sri Lanka among the Therevada tradition around 100 BCE. The second was held by the Sarvastivada school in Kashmir around 100 CE, and had lasting impact on the formation of the Mahayana philosophical traditions.
Therevada Fourth Council
Location: Aluvihare, Sri Lanka - Map Location
Brief Account: It is said that following a year of famine due to poor harvests, many monks whose memories stored the full Pali Canon had passed away. Fearing that this treasury of teachings could be lost if left as only an oral tradition, hundreds of Theravada monks convened in Sri Lanka's Aluvihare Rock Temple to commit these teachings to writing. The process took roughly three years to complete, and the result was a vast collection of inscribed palm leaves compromising the entire Pali Canon.
Sarvastivada Fourth Council
Location: The Fourth Council of the Sarvastivadan tradition is said to have taken place in Kundalavana in the region of Kashmir. The exact location of Kundalavana is unknown, but some scholar think it is adjacent to the modern city of Srinagar in the Kashmir Valley, on the banks of the Jhelum River. - Map Location
Brief Account: During the 1st century CE, Emperor Kanishka of the Kushan empire convened a council in Kashmir to systematize the Abhidharma texts of the Sarvastivadin tradition. This council resulted in the writing of the Maha-vibhasa, an extensive exegesis encompassing all of the Abhidharma teachings in Sanskrit. This text was foundational for the Northern Indian Buddhist traditions, and was the first piece of writing describing the bodhisattva path in conjunction with the path of the shravakas (hearers/arhats) and pratekyabuddhas (solitary realizers). Though this version of Abhidharma describes a part-less particle as the basis for all phenomena, later Mahayana schools expounding the view of emptiness used this view as the basis for their refutations.