PandaX-4T is a 3.7-ton liquid xenon time projection chamber (LXeTPC) operating at the China Jinping Underground Laboratory. An initial search for weakly interacting massive particles (WIMPs) has been recently completed with the commission data, leading to a tightest constraint to WIMP-nucleon interaction. On the other hand, light dark matter has attracted increasing attentions by the community recently, as a lot parameter space has yet to be explored. With the PandaX-4T commissioning data, this work searches for the absorption signal of a galactic sub-MeV dark matter (χ) via the channel χe → eν with an out-going active neutrino, where the mass of χ contributes to the recoiling energy of electrons. The keV-scale sterile neutrino is a representative of such a light fermionic DM, which has already been tightly constrained by astronomical X-ray observations. We found that if χ interacts with electrons and active neutrinos (ν) with an effective new interaction, PandaX-4T can be particularly sensitive to that with an axial-vector or vector mediator.