© 2018 authors. Published by the American Physical Society. We discuss the experimental prospects for observing processes which violate lepton number (ΔL) in four units (or more). First, we reconsider neutrinoless quadruple beta decay, deriving a model independent and very conservative lower limit on its half-life of the order of 1041 ys for Nd150. This renders quadruple beta decay unobservable for any feasible experiment. We then turn to a more general discussion of different possible low-energy processes with values of ΔL≥4. A simple operator analysis leads to rather pessimistic conclusions about the observability at low-energy experiments in all cases we study. However, the situation looks much brighter for accelerator experiments. For two example models with ΔL=4 and another one with ΔL=5, we show how the LHC or a hypothetical future pp collider, such as the FCC, could probe multilepton number violating operators at the TeV scale.