is it characteristic of 4-fixes to feel insecure around "not knowing the self" but accepting that they come off as bland irl anyways? ex. feeling like the Unique people are exploding in colour while you're more of a ghost. i have this theory that john green is 6 core with a 4 fix which is why he romanticises Manic Pixie Dream Girl types
Even as a fix, the 4 places a certain importance to the sense of wanting unicity or being somehow special. So, I don't see a 4 fixer blatantly accept being plain normal. Also, if it is a fix, that kind of insecurity is very much tempered and act more as an "extension fear" of the core fear. In other words, the 4 fixer will have insecurity toward its identity, toward the lack of self knowledge only because it triggers the core fear. Otherwise, if this fear is omnipresent, it would rather indicate a shame core rather than a fix.
However, I could see a 6 core affects that sense of self in that sense of "normal" or dull. Beware though that what I am about to share is more theoretical than observed.
It wouldn't surprise me to see a 6 go into a "I am just bland and ordinary person" mindset because 6s often tend to mainain crappy perception of themselves, even at average level of health. Due to their constant inner doubt, they struggle to build confidence and therefore always destabilize their self esteem.
I would precise that in a broader way seeing yourself as ordinary and normal is not necessarily a horrible perception of yourself, but it is when you innerly seek your unicity and individuality. Since it means you are straight up ignoring one of your inner need. That kind of contradiction and denial systematically results in psychological problems.
Therefore, for a 6 core 4 fix, it would likely be as a way to inflict themselves some sort of degrading treatment and to maintain their expectations and fears low. Since, if they are normal, then at least they cannot lose that unicity as they never had it. But, it still privates them from a sense of worthiness, resulting in a constant conflict between the Self and Ideal Self. Because, if they have a 4 fix, it means they still deep down strive a unique identity, or at least that their definition of identity is constructed around a need for unicity. Therefore, strapping themselves such a label would definitely cause a constant hurt and nourish that perpetual negativity they tend to entertain about themselves when stuck in their core patterns.
Also, because their sense of identity wouldn't be their priority (head core vs shame core), it would be even easier to hold that perception of normalcy and still be functional, despite the conflict it would create. Since the sense of identity supports the 6 core goals rather than being the main goal. Seeing themselves as normal could be their unique way to maintain supportive relations and keep shut their doubting mind. But, again, I insist a part of them would still believe or try to define themsleves in unique ways.
Though, I think the IVs matters a lot to construct such a mindset in a 6 core 4 fix type. I believe Sx blinds 64x or 6x4 would be more likely to entertain the idea of normalcy about their identity, since Sx blinds tend to see the self in terms of a bigger whole at the cost of the individual aspects of the self. Not that they can't see their individuality or other's of course, especially with a 4 fix or if a Fi user. But, since their instinct priority is build around conserving and preserving, and diffusing without discrimination to allow quantity of conncetions, they therefore loose the component of "selecticity" from Sx. The self is therefore build to facilitate preservation and diffusion rather than around its "exclusive" components.
Anyway... I hope it could answer your question. Know that I could totally be wrong, I was pretty hypothetical and theoretical in my answer. I haven't met Sx blind 64x/6x4 much, so I have almost nothing to back up my says else than what "would make sense or not" according to enneagram theory.
Any 64x/6x4 sx blind or of any variant's view would be very welcome. I'd like to know too if I am not saying pure Ni-Ti bs.