(x-4)^(3/2)

There isn't much we can do about it to simplify the expression, unless we are given the value of x, in which case we can evaluate it numerically.

We can also transform it to
(x-4)^(3/2)
=(x-4)^(1+1/2)
=(x-4)√(x-4)
but that is not simpler.

It all depends on which math module you're learning.