  In this work we calculate the corrections to the amputated Green's functions
of 4-fermion operators, in 1-loop Lattice Perturbation theory. One of the novel
aspects of our calculations is that they are carried out to O(a^2) (a: lattice
spacing). We employ the Wilson/clover action for massless fermions (also
applicable for the twisted mass action in the chiral limit) and a family of
Symanzik improved actions for gluons. Our calculations have been carried out in
a general covariant gauge. Results have been obtained for several popular
choices of values for the Symanzik coefficients. While our Green's function
calculations regard any pointlike 4-fermion operators which do not mix with
lower dimension ones, we pay particular attention to DF=2 operators, both
Parity Conserving and Parity Violating (F: flavour). We compute the
perturbative renormalization constants for a complete basis of 4-fermion
operators and we study their mixing pattern. For some of the actions considered
here, even O(a^0) results did not exist in the literature to date. The
correction terms which we calculate are essential ingredients for minimizing
the lattice artifacts which are present in non-perturbative evaluations of
renormalization constants with the RI'-MOM method. Our perturbative results,
for the matrix elements of DF=2 operators and for the corresponding
renormalization matrices, depend on a large number of parameters: coupling
constant, number of colors, lattice spacing, external momentum, clover
parameter, Symanzik coefficients, gauge parameter. To make these results most
easily accessible, we have included them in the distribution package of this
paper, as an ASCII file named: 4-fermi.m; the file is best perused as
Mathematica input. The main results of this work have been applied to improve
non-perturbative estimates of the B_K-parameter in N_F=2 twisted mass lattice
QCD.
