He cant...Im sure all idols must be played by 5...his unplayed idol will be a souvenir. Tony still needs to ensure he wins immunity at 4, which he has been unable to do so far, or ensure that those with him want him at final 3.
rules:
Hidden immunity idols are pocket-sized necklaces made to fit the theme of the season—that are hidden around the tribes' camps or other locations that the castaways have access to. When a castaway finds one of the idols, they have the option of concealing the fact that they have it, or strategically revealing the fact to other castaways. The idols can provide a one-time immunity to a castaway at Tribal Council, if played—which they can be, typically, anytime before the third-to-last Tribal Council. For example, they can be played during the tribal council of the final five castaways in a season where the final tribal council will consist of three players.
http://en.wikipedia.org/wiki/Survivo...immunity_idols