The biggest difference would probably be the INFP's values. Fours are individualistic and need to be unique whereas Nines like to blend in so that they can get along with people. Going by that, I would imagine that Fours are guided by values aimed at self-growth and expression while Nines focus on broader issues like peace and freedom.
[ Ni > Ti > Fe > Fi > Ne > Te > Si > Se ][ 4w5 sp/sx ][ RLOAI ][ IEI-Ni ]
INFP 4s tend to excel at creativity in expression of emotion. Know an INFP that wins people over with art, passion, and style? They are probably a 4. INFP 4s can have vibrate personalities and be very exciting. The downside is that they are dramatic, and if they get into a negative rut will resist fixing that rut, as they identify themselves with their negative emotions and taking action to fix them is a form of self-betrayal.
INFP 9s tend to have fine tuned the art of emotional stability. Which isn't to say they aren't as emotional, but rather have developed a myriad of tactics, both healthy and unhealthy, to be emotionally calm in a storm. 9s are more likely to delve into what makes other people tick, and use that knowledge to create an environment that fosters peace, harmony, and prosperity. The price is that 9s can lose their unique personality, becoming who they need to be to best suit an environment. 9s are also likely to delude themselves and go to far greater lengths to avoid intense, emotional conflict, thereby letting the needs of others completely overwhelm their own personal needs.
One of the issues I've run in to is that I relate to the points of integration and disintegration of the 8, but I'm pretty obviously a 9 in how I act. But I'm very balanced between 8 and 9... it kinda throws me off. I've had a LOT of confusion between 8w9 and 9w8.
Originally Posted by phthalocyanine
i think maybe 4's rely more on Fi and 9's on Ne, in social settings anyway