Viharas were for the purpose of living, Chaityas were assemblies for the purpose of discussions. Further, Chaityas were with Stupas, Viharas did not have stupas.
First Rajgir, Second vaishali, Third patliputraFourth Buddhist Council is the name of two separate Buddhist council meetings first in Sri Lanka and another at Sarvastivada school, in Kashmir around the First Century AD.
EXAMIANS