  We report results of a search for dark-matter-nucleon interactions via a dark
mediator using optimized low-energy data from the PandaX-4T liquid xenon
experiment. With the ionization-signal-only data and utilizing the Migdal
effect, we set the most stringent limits on the cross section for dark matter
masses ranging from 30~$\rm{MeV/c^2}$ to 2~$\rm{GeV/c^2}$. Under the assumption
that the dark mediator is a dark photon that decays into scalar dark matter
pairs in the early Universe, we rule out significant parameter space of such
thermal relic dark-matter model.
