Final answer:
The question involves complex properties and an integral concerning differentiable functions f(x) and g(x). It asks for the value of k, given certain conditions. However, there is insufficient context or information to derive k from the provided excerpts; therefore, a confident and accurate answer cannot be provided.
Step-by-step explanation:
The student's question involves the equalities concerning two differentiable functions f(x) and g(x) over the real numbers, with the given properties and integral expression. We are given that g(f(0)) = 1 and asked to find the value of k in the expression g(f(4)) = e-4k, where k is a natural number. From the provided integral, we understand that the area under the curve f(g(x)) from 0 to a equals 1 - e-2a/2 for all a in the real numbers.
However, the integral and the equation xg(f(x))f'(g(x))g'(x) = f(g(x))g'(f(x))f'(x) are not immediately applicable to finding k without additional context or information on function properties, making it challenging to derive k directly. Thus, we cannot confidently provide an answer to this question as the connection between the given integral and the value of g(f(4)) is not clear based on the supplied excerpts.