  The 4th generation of fermions predicted by the phenomenology of heterotic
string models can possess new strictly conserved charge, which leads, in
particular, to the hypothesis of the existence of the 4th generation massive
stable neutrino. The compatibility of this hypothesis with the results of
underground experiment DAMA searching for weakly interactive particles of dark
matter and with the EGRET measurements of galactic gamma--background at
energies above 1 GeV fixes the possible mass of the 4th neutrino at the value
about 50 GeV. The possibility to test the hypothesis in accelerator experiments
is considered. Positron signal from the annihilation of relic massive neutrinos
in the galactic halo is calculated and is shown to be accessible for planned
cosmic ray experiments.
