With the help of the modified four-particle cluster model Hamiltonian [GJSFR, 10, 18(2010)], the theoretical expressions for the relaxation rate of polarization fluctuation have been evaluated and discussed for the KH2PO4 (KDP)- type crystals by using the double-time thermal dependent retarded Greenâ€™s functions techniques and Dysonâ€™s equation treatment. The correlations appearing in the dynamic equation have been evaluated using double-time thermal dependent retarded Greenâ€™s functions techniques and Dysonâ€™s equation treatment. Without any decoupling the higher order correlations have been evaluated using the renormalized Hamiltonian. By fitting model values in the theoretical expressions, temperature dependence of relaxation rate is calculated for the KH2PO4 (KDP)-type crystals. Theoretical results are compared with experimental result of Garland and Novotny [1969 Phys Rev. 177 971]. A good agreement has been found.