Evaluate the integral of $\int_0^\infty \frac{k}{(1+x)^4} \, dx$

Question

Evaluating the integral:

$$u=(1+x), \qquad du=1\,dx$$

$$\int_0^\infty \frac k {\frac{u^{-4+1}}{-4+1}} \, du \to \left. \frac{-3k}{(1+x)^{-3}} \right|_0^\infty$$

but the answer given has:

$$\left.\frac{-1}{3(1+x)^3} \right|_0^\infty$$ What did I do wrong?