The given point is (8,4) which means we know f(8) = 4.
We don't know any outputs for f(4x) other than the output of 4 which was stated with f(8) = 4. So f(4x) = 4 for some input x. In order for 4x to be equal to 8, that x value must be 2.
f(4x) = f(8)
f(4*2) = f(8)
So the point (2,4) is on the graph of f(4x)
Plugging in x = 2 leads to f(4x) = f(4*2) = f(8) = 4
