  It has been recently pointed out that any primary galactic population of
Weakly Interacting Massive Particles (WIMP) generates, through collisions with
solar matter, a secondary population of ``slow'' WIMPs trapped in the inner
solar system. We show that taking into account this ``slow'' solar-system
population dramatically enhances the possibility to probe the existence of
stable massive neutrinos (of a 4th generation) in underground neutrino
experiments. Our work suggests that a reanalysis of existing underground
neutrino data should be able to bring extremely tight constraints on the
possible existence of a stable massive 4th neutrino.
