  Features of biphoton polarization-frequency ququarts are considered. Their
wave functions are defined as functions of both polarization and frequency
variables of photons with the symmetry obligatory for two-boson states taken
into account. In experiments, biphoton ququarts can display different features
in dependence on whether experiments involve purely polarization or
(alternatively) polarization-frequency measurements. If in experiments one uses
only polarization measurements, the originally pure states of ququarts can be
seen as mixed biphoton polarization states. Features of such states are
described and discussed in details. Schemes of coincidence measurements for
reconstruction of the ququart's parameters are suggested and described.
