Four-year-old brooks is afraid of the sound of the loud train whistle and of going to bed without a light on. his excessive fears are an expression of:

I believe the answer is: immature development of his prefrontal cortex.

Prefrontal cortex of our brain granted us the ability to make logical decision to properly address the situation at hand. 
From brooks' action, we can see that the fear that experienced by brook is illogical and baseless since there is no clear justification of  how he feels the threat.