  We report on an ongoing non-perturbative computation of RI-MOM scheme
renormalization constants for the lattice action with four dynamical flavours
currently in use by ETMC. For this goal dedicated simulations with four
degenerate sea quark flavours are performed at several values of the standard
and twisted quark mass parameters. We discuss a method for removing possible
O(a) artifacts at all momenta and extrapolating renormalization constant
estimators to the chiral limit. We give preliminary results at one lattice
spacing.
