Graphing y = 4 and the focus, we clearly see that the equation we need has the form (x - h)^2 = 4a(y - k).
We need to find a, h and k.
The focus is half way between the vertex and directrix.
You know that y = 4 is 2 units away from the focus and the focus is 2 units down from the focus. So, our vertex is (-3, 0).
From the vertex to the directrix, there are 4 units. Half that distance is the value of a. So, a = 2.
We have all that is needed to form our equation.
(x - (-3))^2 = 4(2)(y - 0)
(x + 3)^2 = 8y
Done.