What is the significance of number 4 in Murder in the Cathedral?

Perhaps the real answer has to do with the four knights who come to the Cathedral and choose to kill Thomas believing they are doing what the king wants. While the 4 Tempters show us something about Thomas, they simply teach us more about his moral character and his ability to say no to the Temptations of the Earth (1-3) and the Temptations of the After Life (4). The knights show the fact that 4 against 1 is an unfair fight, and their attack and murder indeed elevate him to the status of a martyr.