My experience is that it's related to, but distinct from, g. High g and more mature age make the higher levels easier but don't create them on their own.
Why would a high-IQ level 4 person have trouble emulating level 5? See e.g. Sokal, etc.
ETA: I looked through the linked article and I stick by my impression that this is a straightforward IQ ladder modified by "maturity" (appropriate socio-emotional development, I guess?) In particular, I expect that levels have pretty hard IQ requirements, e.g. a person with the IQ of 80 just won't make it to Level 4.
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