  Finite-temperature phase transitions in quasi-one-dimensional quarter-filled
systems are investigated by the extended Hubbard model with electron-lattice
coupling. Using a quantum Monte Carlo method combined with the inter-chain
mean-field approximation, we clarify competing and coexisting behaviors among
charge ordering, dimer Mott, and spin-Peierls states. It is pointed out that an
anharmonicity of lattice distortions plays an important role in multi-critical
behaviors. The results are compared with experimental data for
quasi-one-dimensional molecular conductors such as DCNQI and TMTTF compounds.
