Find the value

$\quad \quad \Gamma(n) = (n-1)! = \displaystyle\int_{0}^{\infty} e^{-x}x^{n-1} \cdot dx \\ \quad \quad \Gamma(4) = \boxed{3! = 6 = \displaystyle\int_{0}^{\infty} e^{-x}x^{3} \cdot dx} \\$