I am assuming you are referring to the play Murder in the Cathedral, not the Jean Anouilh’s version Becket. In this play, Thomas Becket does reply to the fourth tempter - after a short hesitation. This tempter is tempting him to commit what he believes is a grievous sin. To seek to become a martyr in order to gain personal glory is the worst example of pride a man can have. Becket believes that if he is to be a martyr, it is by God's will. He tells the tempter:
The last temptation is the greatest treason: to do the right deed for the wrong reason.
The fourth temptation, therefore, would be the worst sin in Becket's mind. He does not care for his physical safety (first temptation), he has already served the king and has power (second temptation), and a coalition against the king would be political treason which he does not want to commit. But treason against God would be the worst.
If this is not what you are asking, you can read the analysis for both Becket and Murder in the Cathedral here on eNotes.