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.
