\begin{align*}\int_{0}^{\infty}\frac{ x^3}{e^x-1} \,dx &= \int_{0}^{\infty} \frac{ x^3e^{-x}}{1-e^{-x}} \,dx = \int_{0}^{\infty} x^3 e^{-x}\sum_{n=0}^{\infty} e^{-xn}\,dx \\&=
\sum_{n=0}^{\infty} \int_{0}^{\infty} x^3e^{-nx-x} \,dx =\sum_{n=0}^{\infty} \frac6{(1 + n)^4} = \frac{\pi^4}{15}\end{align*}