Answer
$$y= \frac{1}{16}x^2 .$$
Work Step by Step
The focus is at $(0,4)$ and the directrix is $y=-4=-c$, hence $c=2$. The parabola is vertical, opens upwards, and has the equation
$$y=\frac{1}{4c}x^2=\frac{1}{16}x^2 .$$
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