Four-loop photon quark form factor and cusp anomalous dimension in the   large-$N_c$ limit of QCD
  We compute the four-loop QCD corrections to the massless quark-anti-quark-photon form factor $F_q$ in the large-$N_c$ limit. From the pole part we extract analytic expressions for the corresponding cusp and collinear anomalous dimensions. 
Johannes Henn, Roman N. Lee, Alexander V. Smirnov, Vladimir A.   Smirnov, Matthias Steinhauser
hep-ph