If $$f(x) = \frac{2x}{1+x^{2}}$$ find $f'( 4 )$.

$f'( 4 )$ =

Use this to find an equation of the tangent line to the curve $y = f(x)$ at the point $\left( 4 , {\frac{8}{17}} \right)$.

An equation of the tangent line is $y$ = .