  A search for low-mass dilepton resonances in Higgs boson decays is conducted
in the four-lepton final state. The decay is assumed to proceed via a pair of
beyond the standard model particles, or one such particle and a Z boson. The
search uses proton-proton collision data collected with the CMS detector at the
CERN LHC, corresponding to an integrated luminosity of 137 fb$^{-1}$, at a
center-of-mass energy $\sqrt{s} =$ 13 TeV. No significant deviation from the
standard model expectation is observed. Upper limits at 95% confidence level
are set on model-independent Higgs boson decay branching fractions.
Additionally, limits on dark photon and axion-like particle production, based
on two specific models, are reported.
