Non-Elementary Integration · Easy problem

Using the Gamma function $\Gamma(s)=\displaystyle\int_0^{\infty} x^{s-1}e^{-x}\,dx$ and the fact that $\Gamma(n)=(n-1)!$ for positive integers $n$, evaluate $\displaystyle\int_0^{\infty} x^{3}e^{-x}\,dx$.