  A possible Standard Model extension of the Pati-Salam type with a lepton
number as the fourth color is reexamined. A new type of mixing in the
interaction of the $SU(4)_V$--leptoquark with quarks and leptons is shown to be
required. An additional arbitrariness of the mixing parameters could allow to
decrease noticeably the lower bound on the leptoquark mass $M_X$ originated
from the $\pi$ and $K$ decays and the $\mu e$ conversion. The only mixing
independent bound emerging from the cosmological limit on the $\pi^0
\rightarrow \nu \bar{\nu}$ decay width is $M_X > 18~TeV$.
