"INFPs who are relying on their Intuition this way (Defensively) usually take one of two directions. Either they become permanent seekers - good at many things but disinclined to stick with any for long - or they become somewhat passive, unable to articulate what they want but dissatisfied with what they're doing." - Personality Type: An Owner's Manual by Leonore Thomson
First, I apologize in not giving the full quote.
But anyhoo, let us assess, students. The latter in the quote relates to 9 behavior I would assume. While the former, I find difficult to interpret - because it seems more 5 behavior.
As Extraverted Intuition is likely how INFP's relate to the world, know that to use it defensively, as such, the INFP would have to be in the grip. Probably to be driven in such by their inferior function, Inferior Thinking (I think). So, mayhaps, we should clearly define what the four different INFP functions are first; then try and interpret how each function might turn out differently (or be used more often) for a 4 and a 9 on the Enneagram.
The reason why we must define the four functions of this INFP personality is because:
A) People are more often to misidentify as E-type 4 then 9. Think of the name 4 is usually given, 'Artist'. Both, and all, can be creative; but as the name implies and can be mistaken. (INFP may be 9 as much as 4.)
B) If we can define each function for people to look at, then identify what they use or have developed. Their experience may be easier to share. (Introverts do not necessarily show themselves at face value as an Extrovert; so they may not identify with the Primary Function.)