  Samples of PbS$_x$Se$_y$Te$_{1-x-y}$, Pb$_{1-x}$Sn$_x$Te$_{1-y}$Se$_y$, and
Pb$_{1-x}$Sn$_x$Te$_{1-y}$S$_y$ quaternary solid solutions were investigated in
the 4--200~K temperature range using electrical and X-ray methods. The regions
where low-temperature phase transitions occur were established. It was shown
that phase transitions in these solid solutions are associated with off-center
S and Sn ions. The dependence of the phase transition temperature on the
composition of solid solutions can be qualitatively described taking into
account the influence of substitutional disorder on the ordering and tunneling
of off-center ions.
