Four-loop QCD $\beta$-function with different fermion representations of   the gauge group
  We present analytical results at four-loop level for the $\beta$-function of the coupling of a generic gauge group and any number of different quark representations. From this we can directly derive the gluino contribution to the strong coupling $\beta$-function of supersymmetric extensions of the Standard Model. 
M. F. Zoller
hep-ph