In the novel The Big Four (and the short stories making up the novel) is the designation given by the British secret service agent Mayerling for the fourth member of the leadership of the highly secretive and powerful internation crime syndicate the Big Four. Of all the four, Mayerling knew least about Number Four. He only knew that he was The Destroyer.
As Poirot would discover, Number Four represented the operational arm of the organisation. Time and again, he was behind efforts to kill anyone who could betray the secrets of the organisation. His methods were varied and creative and he was a master of disguise.
The true identity of Number Four would be the last among the group to be discovered. It is only towards the end of the story arc that Poirot would discover that Number Four was an actor named Claud Darrell.