  We prove in this paper that the electro-weak SU(4)L x U(1)X gauge models with
spontaneous symmetry breaking can offer a natural framework for generating
neutrino masses by simply exploiting the tree level realization of
dimension-five effective operators. The novelty of our approach resides in the
fact that the scalar sector needs not to be enlarged, since these operators are
constructed as direct products among scalar multiplets already existing in the
model. There is a unique generic matrix for Youkawa couplings in the neutrino
sector. The charged leptons are already in their diagonal basis. This framework
can lead to a suitable fit of the established phenomenology for the left-handed
neutrinos, while the right-handed neutrino masses come out in the sub-keV
region, independently of the cut-off. The latter introduces in the theory an
intermediate scale (however, more close to GUT than to SM) at about 10^12GeV
which is a crucial ingredient for the left-handed neutrino phenomenology.
