How to configure geo-redundant hidden replica to become primary?

[Bernardo]

I have a MongoDB replica set in EC2, most of which is located on the East coast.

Two read/write nodes, one hidden zero-priority 4hour delay node for backup purposes, and one arbiter.

I just added a 4th hidden zero-priority node in a West Coast EC2 zone. I would like to prevent any reads from it (hidden) and I would like to prevent it from becoming primary unless the entire East Coast replica is down. Currently with 0-priority it's never going to go primary unless I manually go in and change it.

Suggestions?

I might possibly need to add an arbiter on the West Coast as well?

Thanks!

[Bob Grabar]

Bernardo, Are you still looking for answers, and if so, do you have five members total (2 r/w, 2 hidden/0, 1 arbiter)? -Bob