Within the framework of QCD we compute renormalization constants for the
strong coupling and the quark masses to four-loop order. We apply the
DR-bar scheme and put special emphasis on the additional couplings which
have to be taken into account. This concerns the epsilon-scalar–quark
Yukawa coupling as well as the vertex containing four epsilon-scalars.
For a supersymmetric Yang Mills theory, we find, in contrast to a
previous claim, that the evanescent Yukawa coupling equals the strong
coupling constant through three loops as required by supersymmetry.