The 4 Tempters and the 4 Knights may or may not be played by the same actors, but they are quite different. Just as the devil does when Jesus goes out into the wilderness, so also the Tempters come to Thomas Becket at the beginning of the play to offer him all sorts of incentives to recant his rulings on the king. Thomas easily dismisses three of the Tempters, but when the fourth offers him sainthood and the fame that goes with it, Thomas hesitates for an instant. However, he is able to dismiss all four.
The 4 Knights don't offer any incentives other than violence. If Thomas does not rule in Henry's favor, he will pay with his life. Thomas does not give in, and they murder him. Ironically, by not giving in he earns the sainthood that the 4th Tempter had offered him.