What is more immediate, the Rule or the Action? In other words, does the playing of Inflation close the Box, so to speak; or does it affect the Box, meaning you have to draw to a fourth rule?
A: It goes to 4. Since everything happens immediately in Fluxx, Inflation is applied to the Action that caused it to be played. So yes, you’d continue to a fourth rule if Inflation came up during Pandora’s Box. (Similarly, Draw 2 and Use ’em would instantly become Draw 3 and Use ’em if Inflation was one of the two cards drawn.)
Adventure Time includes the Inflation analogue, Mathematical! Inflation is also available as a promo card to add to any Fluxx deck.