  Over the last several years, our understanding of neutrino oscillations has
developed significantly due to the long-baseline measurements of muon-neutrino
disappearance and muon-to-electron-neutrino appearance at the T2K and NOvA
experiments. However, when interpreted under the
standard-three-massive-neutrinos paradigm, a tension has emerged between the
two experiments' data. Here, we examine whether this tension can be alleviated
when a fourth, very light neutrino is added to the picture. Specifically, we
focus on the scenario in which this new neutrino has a mass similar to, or even
lighter than, the three mostly-active neutrinos that have been identified to
date. We find that, for some regions of parameter space, the four-neutrino
framework is favored over the three-neutrino one with moderate (a little under
two sigma) significance. Interpreting these results, we provide future outlook
for near-term and long-term experiments if this four-neutrino framework is
indeed true.
