  The first complete calculation of the next-to-leading-order electroweak
corrections to four-lepton production at the LHC is presented, where all
off-shell effects of intermediate Z bosons and photons are taken into account.
Focusing on the mixed final state $\mu^+\mu^-e^+e^-$, we study differential
cross sections that are particularly interesting for Higgs-boson analyses. The
electroweak corrections are divided into photonic and purely weak corrections.
The former exhibit patterns familiar from similar W/Z-boson production
processes with very large radiative tails near resonances and kinematical
shoulders. The weak corrections are of the generic size of 5% and show
interesting variations, in particular a sign change between the regions of
resonant Z-pair production and the Higgs signal.
