It is interesting that you have asked this. Quite recently I have attempted to measure cognitive complexity among fifth graders. I have realized that it is better to increase the tasks (one by one) that individual students are engaged in and abserve at what point each student finds it difficult to cope intellectually. I am observing each student separately because of individual differences in cognitive complexity. As such a mean score becomes less useful given the fact that individual students learn at different rates. 

We may not know the cognitive complexity score of a 4th grade reading level, but someone did an analysis comparing the average 4th grade reading level to Trump's. Near perfect match.

http://www.newsweek.com/trump-fire-and-fury-smart-genius-obama-774169

Whether or not we can find out the cognitive complexity score of children would not make any difference in their ability to learn or their socio- emotional development...what then is the objective of such an exercise given that the acquisition of knowledge should serve always serve a practical purpose?

I think I might respectfully disagree here. If a reading level is associated with a lower or higher degree of nuance, which is what the cognitive complexity scoring system measures, then reading that attempts to match that level of nuance could be developed (and possibly) incrementally increased so that child's "scores" increase.

But I agree that socioeconomic status and the trauma of poverty, attention span and other factors must be considered as well. Its more supplementary than diagnostic.

I think that determining the degree of cognitive complexity of the average level of fourth-grade reading depends on several things, including: the economic and social level of the country, the nature of the curricula offered to students, the level of teacher performance, as well as the educational environment.

This file give you a large view about cognitive complexity of reading:

https://files.eric.ed.gov/fulltext/ED603577.pdf