A series of liquid crystalline mixtures that exhibit remarkably stable 4-layer intermediate phases, with phase ranges of around 30 degrees C, are reported. The mixtures are formed from a selenium-containing antiferroelectric material combined with up to 9% ( by weight) of a highly chiral dopant. Various experimental methods including X-ray scattering and electro-optic techniques were employed in order to determine a set of physical parameters for each of the mixtures, such as smectic layer spacing, steric and optical tilt angles and spontaneous polarization. It is shown that the antiferroelectric phase is lost at dopant concentrations between 5 and 6%, when the stability of the 4-layer intermediate phase becomes significantly enhanced. This transition is associated with a decrease in the ratio of the spontaneous to flexoelectric coupling coefficients in the system. The combined observations suggest that molecular conformation is a more significant factor in stabilising the intermediate phases than chirality. The data are compared with existing theories of the intermediate phases and some, but not complete, agreement is observed.