I'm not exactly sure if you want the other three answers to remain after the fourth is displayed, but I think I may still be able to help. Also, I'm not sure if you want the fourth choice to be seen and just not selectable.
Just make the third choice display another set of choices in which the first three have the same result, but adding a fourth. If you want the choices to keep popping up after the NPC answers the questions, create a loop, and set it so that if the player cancels, it breaks the loop.
I hope that helps/makes sense