  The proton ordering model for the KH$_{2}$PO$_{4}$ type ferroelectrics is
modified by taking into account non-linear effects, namely, the dependence of
the effective dipole moments on the proton ordering parameter. Within the
four-particle cluster approximation we calculate the crystal polarization,
longitudinal dielectric permittivity, specific heat, and explore the
electrocaloric effect. Smearing of the ferroelectric phase transition by the
longitudinal electric field is described. A good agreement with experiment is
obtained.
