Many quasi-one-dimensional organic crystals, known as synthetic metals at room temperature, become insulators when the temperature decreases, due to a Peierls transition. In the crystals with half-filled Brillouin zone the dimerization of lattice takes place at some critical temperature that determines the phase transition. The Peierls structural transition in tetrathiotetracene iodide (TTT2I3) crystals with a quarter filled zone is studied. A more complete crystal model is used which takes into account two main electron-phonon interactions. The polarization operator is calculated in two cases: when impurities are absent and when the scattering on impurities takes place.