[exim] router question

Hi all...

Basically, lets say I have router1, router2, router3, router4, etc in
this order.
If the precondition on router1 is OK, I want to try router3 (instead of
sending to tranport), but if NOT OK I want to router4.

I see there is a pass_router in the docs, but I'm not sure how to use that.

Thanks