What are the other Theravada Councils held between the 4th and 6th
What are these other councils? Where were they held? What was their significance?

I am going to try to answer this to the best of my abilities.

I do not know the specifics of where they were held, but I do know what they are &amp; their significance.

All the Buddhist councils are meant to preserve the Dhamma the Buddha formulated &amp; to discuss the Tipitaka &amp; its interpretations. 

Buddhist councils have not consisted of all schools unified, although in some cases they have been. 

I hope my answer suits you well! :) 

May you be well.