The inverse function of the above function is f(x) = 2x - 8 and therefore the function at 4 would be equal to 0.
You can find the inverse of any function by switching the f(x) and x terms. Once you have done that, solve for the new f(x). Finally, what you'll have remaining is the inverse function. The work is done for you below:
f(x) = 1/2x + 4 ----> Switch the x and f(x)
x = 1/2f(x) + 4 ---> subtract 4 from both sides
x - 4 = 1/2f(x) ----> multiply both sides by 2
f(x) = 2x - 8
This would be your inverse function. Now to find f-1(4) you would put 4 in for x in your new inverse function.
f(x) = 2x - 8
f(4) = 2(4) - 8
f(4) = 8 - 8
f(4) = 0