  We reconsider the possibility that Majorana masses for the three known
neutrinos are generated radiatively by the presence of a fourth generation and
one right-handed neutrino with Yukawa couplings and a Majorana mass term. We
find that the observed light neutrino mass hierarchy is not compatible with low
energy universality bounds in this minimal scenario, but all present data can
be accommodated with five generations and two right-handed neutrinos. Within
this framework, we explore the parameter space regions which are currently
allowed and could lead to observable effects in neutrinoless double beta decay,
$\mu - e$ conversion in nuclei and $\mu \rightarrow e \gamma$ experiments. We
also discuss the detection prospects at LHC.
