Answer
$${y^2} = 16x$$
Work Step by Step
$$\eqalign{
& {\text{focus}}\left( {4,0} \right) \cr
& {\text{The focus is }}\left( {p,0} \right),{\text{ then the equation is of the form }} \cr
& {y^2} = 4px \cr
& {\text{with }}p = 4 \cr
& {y^2} = 4\left( 4 \right)x \cr
& {y^2} = 16x \cr} $$
