  All the possible schemes of neutrino mixing with four massive neutrinos
inspired by the existing experimental indications in favor of neutrino mixing
are considered in a model independent way. Assuming that in short-baseline
experiments only one mass-squared difference is relevant, it is shown that the
scheme with a neutrino mass hierarchy is not compatible with the experimental
results. Only two schemes with two pairs of neutrinos with close masses
separated by a mass difference of the order of 1 eV are in agreement with the
results of all experiments. One of these schemes leads to possibly observable
effects in Tritium and neutrinoless double-beta decay experiments.
