This sounds a lot like the theory of crystallized vs fluid intelligence: https://en.wikipedia.org/wiki/Fluid_and_crystallized_intelligence
As far as I know, by most any commonly used metric, both of these will increase well beyond four years of age. Vaniver mentions 15 years old, and I recall 19-20 years old being the number given for maximum fluid intelligence in the psychology textbook I had in undergrad.
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