You require "statistical calculations" to support the notion that a sample size of 4000 people in two countries, two of the most modernized and affluent nations on the planet at that, is too small for a study to conclude how the human brain works across billions of people alive today across the world, Much less in human history? How about common sense?

Ok, let's do some math.

Approximately 108 to 117 billion humans are estimated to have lived in human history. Let's take 110 billion (the low end) for our purposes.

4,000 / 110,000,000,000 = 0.000003636%

Let's just go with people living today, which is approximately 8 billion (the low end).

4000 / 8 billion = 0.00005%

Not sure if that covers the "statistical" part of your requirement, but it covers the "calculations" part.

I myself would be hesitant to make claims about knowing how neurons and brains and ages work based on a sample size of pessimistically 0.000003636% or even optimistically 0.00005% of the human population and their brains.