  We study possible current and future low-energy signals of the gauge
leptoquark in quark-lepton SU(4) unification \`a la Pati-Salam. Taking fully
into account the freedom in the generation mixing between quarks and leptons,
we compile a catalogue of observables which currently form a border of the
excluded part of the parameter space - hot candidates for first signals of new
physics. We also determine the sensitivity needed in order to inspect a
currently allowed part of the parameter space for several other measurements
which are not included in this catalogue. We improve older similar works on
this topic by taking into account more (and more recent) experimental
measurements and by scanning the parameter space more densely.
  Furthermore, we study in a similar manner the SU(4) models with a small
number of generations of extra leptons. We also discuss the minimal number of
leptons needed in order to alleviate the contemporary discrepancies in the
neutral-current B-meson decays.
