Let $x = 4 \cos \theta$ $0 \leq \theta < \frac { \pi } { 2 }$

Let $x = 4 \cos \theta$ , $0 \leq \theta < \frac { \pi } { 2 }$ . Simplify the expression. $\frac { 4 x } { \sqrt { 16 - x ^ { 2 } } }$