  This is the third of a series of papers on three-loop computation of
renormalization constants for Lattice QCD. Our main point of interest are
results for the regularization defined by Iwasaki gauge action and n_f=4 Wilson
fermions. Our results for quark bilinears renormalized according to the RI'-MOM
scheme can be compared to non-perturbative results. The latter are available
for Twisted Mass QCD: being defined in the chiral limit, renormalization
constants must be the same. We also address more general problems. In
particular, we discuss a few methodological issues connected to summing the
perturbative series such as the effectiveness of Boosted Perturbation Theory
and the disentanglement of irrelevant and finite volume contributions.
Discussing these issues we consider ont only the new results of this paper, but
also those for the regularization defined by tree-level Symanzik improved gauge
action and n_f=2 Wilson fermions, which we presented in a recent paper of ours.
We finally comment to which extent the techniques we put at work in the NSPT
context can provide a fresher look into the lattice version of the RI'-MOM
scheme.
