A theoretical study was carried out on the ionicity and transverse effective charge (e(T)*) in GaxIn1-xAsySb1-y quaternary system lattice matched to GaSb and InAs for the zinc blende phase. The calculations are based on the pseudopotential theory under the virtual-crystal approximation. The expression we used for determining e(T)* provides good agreement with available experimental data. Attention has been paid to the influence of the alloy disorder on the studied quantities by taking into account in the calculations the effects of compositional variations. The effect of changing the substrate is also checked. Both effects are found to have an influence on the results.