  We study $\mu - e$ conversion with sequential four generations. A large mass
for the fourth generation neutrino can enhance the conversion rate by orders of
magnitude. We compare constraints obtained from $\mu - e$ conversion using
experimental bounds on various nuclei with those from $\mu \to e \gamma$ and
$\mu \to e\bar e e$. We find that the current bound from $\mu - e$ conversion
with Au puts the most stringent constraint in this model. The relevant flavor
changing parameter $\lambda_{\mu e} = V^*_{\mu 4}V_{e4}^{}$ is constrained to
be less than $1.6\times 10^{-5}$ for the fourth generation neutrino mass larger
than 100 GeV. Implications for future $\mu -e$ conversion, $\mu \to e\gamma$
and $\mu \to e\bar e e$ experiments are discussed.
