Scheme dependence and the NSVZ $\beta$-function
  We investigate the connection between the NSVZ and the DRED forms of the gauge $\beta$-function in an $N=1$ supersymmetric gauge theory. We construct a coupling constant redefinition that relates the two forms up to four loops. By abelian calculations, we are able to infer the complete non-abelian form of $\beta_g^{(3)DRED}$, and also $\beta_g^{(4)DRED}$ except for one undetermined parameter. 
I. Jack, D.R.T. Jones and C.G. North