  Many Standard Model extensions can contribute to four-lepton signals at large
colliders. We review the particular case of leptophilic interactions eventually
observable at the LHC and the ILC, paying special attention to the addition of
a new vector boson coupled to muon minus tau lepton number, $Z'_{\mu - \tau}$,
and emphasizing the prospects at a very large hadron collider with $\sqrt s =$
100 TeV. We also discuss in this case the new contribution to two-lepton
(Drell-Yan) production when the new leptophilic interaction has a non-vanishing
kinetic mixing with the SM.
