The signals tell the defensive story

Sir Winston Churchill said, "The statesman who yields to war fever must realize that once the signal is given, he is no longer the master of policy but the slave of unforeseeable and uncontrollable events."

The defenders, during the war at the bridge table, must signal and hope that each will be interpreted correctly.

This deal came from a reader who requested anonymity. First, what should North rebid over partner's one spade? Second, how should the defenders play assuming South is in four spades, and West leads the diamond ace?