  If a fourth generation of leptons exists, both the neutrino and its charged
partner must be heavier than 45 GeV. We suppose that the neutrino is the
heavier of the two, and that a global or discrete symmetry prohibits
intergenerational mixing. In that case, non-renormalizable Planck scale
interactions will induce a very small mixing; dimension five interactions will
lead to a lifetime for the heavy charged lepton of $O(1-100)$ years. Production
of such particles is discussed, and it is shown that a few thousands can be
produced and collected at a linear collider. The possible uses of these heavy
leptons is also briefly discussed.
