We study radiative leptonic decays (\ell^- \to \ell'^- \bar{\nu}{\ell'} \nu\ell \gamma) with massive Dirac and Majorana neutrinos within the general dimension-six four-lepton effective interaction. We derive the fully differential decay distribution, including neutrino mixing and the initial charged-lepton polarization, and express it in terms of generalized Michel parameters. The result is organized by neutrino- and charged-lepton-mass orders and recovers the known massless-neutrino limit. We identify the interference terms sensitive to the neutrino nature and examine their mass and mixing suppression. We further explore scenarios with additional scalar interactions, illustrating their effects on heavy–light contributions to radiative (\tau \to \mu) energy spectra.