  We compute the four-loop $n_f$ contribution proportional to the quartic
Casimir of the QCD cusp anomalous dimension as an expansion for small cusp
angle $\phi$. This piece is gauge invariant, violates Casimir scaling, and
first appears at four loops. It requires the evaluation of genuine non-planar
four-loop Feynman integrals. We present results up to ${\mathcal O}(\phi^4)$.
One motivation for our calculation is to probe a recent conjecture on the
all-order structure of the cusp anomalous dimension. As a byproduct we obtain
the four-loop HQET wave function anomalous dimension for this color structure.
