We study neutrino mass generation in models with four chiral families of
leptons and quarks and four right handed neutrinos. Generically, in these
models there are three different contributions to the light neutrino masses:
the usual see-saw contribution, the tree-level contribution due to mixing of
light neutrinos with neutrino of the fourth generation, and the two loop
contribution due to the Majorana mass term of the fourth neutrino. We study
properties of these contributions and their experimental bounds. The regions of
the parameters (mixings of the fourth neutrino, masses of RH neutrino
components, etc.) have been identified where various contributions dominate.
New possibilities of a realization of the flavour symmetries in the four family
context are explored. In particular, we consider applications of the smallest
groups, e.g. SG(20,3), with irreducible representation 4.Comment: 33 pages, 4 figures; Eq. (18) corrected and thus corrections to Eqs.
(21,26-28,41,42,44-46) and figures, the loop contribution reduced by 2 orders
of magnitude; general conclusions unchanged; accepted by Nucl. Phys.