INFP here. 9 core, but I have a 4 fix. I've never gotten why people do that with their avis. If I wanna be unique and authentic, I'm not gonna copy everyone else's emo avi and look the same, I wanna do my own thing. Then again, when I'm picking an avi, I don't necessarily try to go for any avis that are expressing sadness. Sometimes I do, but I just try to pick avis that are stylish and express who I am, whatever side of me that is. It's the same things for clothes-the stereotype is that the 4s are wearing all black, when for me I feel like I wouldn't be authentic if I was wearing all black(and it just feels dull to me, but yeah). It's authentic for some people of course, but then there's also some people wanting that 4ish look. I like colorfulness(with some black here and there), and that's what's authentic to me, even if doesn't fit the stereotype.
There could definitely be mistypes for sure, as it seems attachment types are the most common(and whaddya know, I got 2). Fi definitely looks 4ish as it doesn't wanna adhere to the typical and withdraws into itself to find their true emotions. But I do that too as a 9 first; I do suppress a lot of stuff to keep the peace, but I'm a bit stubborn because I try to adhere to my values. I feel like 9 as a withdrawn type too spends a lot of time in their head processing their feelings because they need to make sense of things. Identity searching would appeal to them too, because they're such a merging and ambivalent type that having an identity would make them feel like somebody. That's what it feels like for me-I do relate to the 4 fear of having no identity for well, 4ish reasons, but also I look for identity because I merge so easily with my surroundings and don't quite fit anywhere(one of the reasons why I like typology so much).