Answer
$f(x)= \frac{x}{x+4}$
And:
$g(x)=x^{2}$
Other answers are also possible.
Work Step by Step
We want: $G(x)=(f\circ g)(x)=f(g(x))$.
We make:
$f(x)= \frac{x}{x+4}$
And:
$g(x)=x^{2}$
Then:
$f(g(x))=\frac{x^{2}}{x^{2}+4}=G(x)$
Other answers are also possible.