Thermodynamic investigations by heat capacity measurements at low temperature between 700mK and 60K with magnetic fields up to 8T for θ-(BEDT-TTF)2RbZn(SCN)4 and θ-(BEDT-TTF)2CsZn(SCN)4, where BEDT-TTF is bisethylenedithiotetrathiafulvalene are performed. In θ-(BEDT-TTF)2RbZn(SCN)4, the inter-site Coulomb repulsion produces a charge ordered ground state in the case of slow cooling while a kind of charge cluster glass state appears in rapidly cooled case. The observed finite γ term of 14.8mJK−2mol−1 for the rapidly cooled sample demonstrates that the electron density of states remains as metal-like clusters which are localized in the short-range scale. We also discuss the enhancement of the lattice heat capacity and appearance of a kind of Boson peak structure in Cp even in a regular crystal lattice in θ-(BEDT-TTF)2CsZn(SCN)4. The low energy phonon modes produced by enhanced charge fluctuations due to the inter-site Coulomb repulsion are the main course of these unusual lattice properties. The features reported here suggest that the coupling of charge degrees of freedom with lattices is important for the systems which do not form rigid charge ordered structures.