Primitive of x cubed over x fourth plus a fourth/Proof 2

Theorem

 * $\displaystyle \int \frac {x^3 \ \mathrm d x} {x^4 + a^4} = \frac {\ln \left({x^4 + a^4}\right)} 4$

Proof
From Primitive of Power of x less one over Power of x plus Power of a:
 * $\displaystyle \int \frac {x^{n - 1} \ \mathrm d x} {x^n + a^n} = \frac 1 n \ln \left\vert{x^n + a^n}\right\vert + C$

So: