Question : External DNS Redundancy

We have 4 external DNS servers for A record lookups for www and mx record lookups for email..  2 DNS servers are in one City 2 are in another.

If 2 go down, the other 2 should be sufficient to handle queries.

I am looking for supporting documentation which shows how a resolver will be able to go through the list of DNS servers on behalf of the client and not timeout before finding a server that is not down.

Please comment and provide a link to how a resolver goes thru a list of Name Servers if if does not receive an answer from the first Name Server.

Thanks

Answer : External DNS Redundancy

Still no takers?
Any External DNS Experts out there?

Thanks