  We consider extensions of the Standard Model by vectorlike leptons and set
limits on a new charged lepton, $e_4^\pm$, using the ATLAS search for anomalous
production of multi-lepton events. It is assumed that only one Standard Model
lepton, namely the muon, dominantly mixes with vectorlike leptons resulting in
possible decays $e_4^\pm \to W^\pm \nu_\mu$, $e_4^\pm \to Z\mu^\pm$, and
$e_4^\pm \to h \mu^\pm$. We derive generally applicable limits on the new
lepton treating the branching ratios for these processes as free variables. We
further interpret the general limits in two scenarios with $e_4^\pm$
originating predominantly from either the $SU(2)$ doublet or the $SU(2)$
singlet. The doublet case is more constrained as a result of larger production
cross-section and extra production processes $e_4^\pm \nu_4$ and $\nu_4\nu_4$
in addition to $e_4^+ e_4^-$, where $\nu_4$ is a new neutral state accompanying
$e_4$. We find that some combinations of branching ratios are poorly
constrained, whereas some are constrained up to masses of more than 500 GeV. In
the doublet case, assuming BR$(\nu_4\rightarrow W\mu) = 1$, all masses below
about 300 GeV are ruled out. Even if this condition is relaxed and additional
decay modes, $\nu_4 \to Z \nu_\mu$ and $\nu_4 \to h \nu_\mu$, are allowed,
below the Higgs threshold still almost all of the parameter space (of
independent branching ratios) is ruled out. Nevertheless, even assuming the
maximal production cross-section, which coincides with the doublet case, the
new charged lepton can still be as light as the LEP-II limit allows. We discuss
several possible improvements of current experimental analyses that would
dramatically reduce the allowed parameter space, even with current data.
