Spontaneous birefringence δsΔn in the INC and C phases of Rb2ZnCl4 crystals has been evaluated and shown to consist of contributions proportional to <r2> and <r6>, r denoting the order parameter modulus. Jump-like changes in Δn of the order of 10-6 are observed at the INC-C transition. It is shown that they can be explained by a change D<r>/<r>\cong0.25% and by the occurrence of another contribution in the INC phase. The latter could originate from the fact that the light propagation may be affected by spatial modulation of optical permittivities. If temperature is cycled in the INC phase only, no hysteresis in Δn is observed. This shows that-unlike static permittivity-optical properties of Rb2ZnCl4 are not sensitive to which is proportional to soliton density.