Textbook Problem

For Problems $1-18$, determine $\left(f\circ g\right)\left(x\right)$.and $\left(g\circ f\right)\left(x\right)$ for each pair of functions. Also specify the domain of $\left(f\circ g\right)\left(x\right)$ and $\left(g\circ f\right)\left(x\right)$. (Objective 1)

$f\left(x\right)=\frac{1}{x}$ and $g\left(x\right)=\frac{1}{x-4}$