  We consider the neutrino sector of a Standard Model with four generations.
While the three light neutrinos can obtain their masses from a variety of
mechanisms with or without new neutral fermions, fourth-generation neutrinos
need at least one new relatively light right-handed neutrino. If lepton number
is not conserved this neutrino must have a Majorana mass term whose size
depends on the underlying mechanism for lepton number violation. Majorana
masses for the fourth generation neutrinos induce relative large two-loop
contributions to the light neutrino masses which could be even larger than the
cosmological bounds. This sets strong limits on the mass parameters and mixings
of the fourth generation neutrinos.
