Isopiestic osmotic coefficients have been determined for the unsaturated and NH4NO3-saturated quaternary systems H2O-NH4NO3-Y(NO3)(3)-Pr(NO3)(3) and H2O-NH4NO3-Pr(NO3)(3)-Nd(NO3)(3) and their subsystems H2O-NH4NO3-Y(NO3)(3), H2O-NH4NO3-Pr(NO3)(3), and H2O-NH4NO3-Nd(NO3)(3) up to the maximum ionic strength I = 30 mol center dot kg(-1) at 298.15 K. As Y(NO3)(3), Pr(NO3)(3), and Nd(NO3)(3) are 3/1 electrolytes and may form complexes (or double salts) with NH4NO3 at high concentrations, their highly unsymmetrical mixing up to NH4NO3 saturation throws light on the importance of ionic interactions and solute-solute interactions in such complicated solutions. A modified ion interaction (Pitzer) model extending to C(3) and ohm(cc'5X) has been presented, which enables the new osmotic coefficient data for the ternary and quaternary systems and the literature data for their other subsystems to be represented quantitatively in the whole concentrations, where the C(3) parameter is specific to 3/1 rare earth electrolytes while the ohm(cc'5X) parameter belongs to the (Ln(3+))(NH4+)(NO3-)(5) interactions. The solute-solute interactions have been analyzed for the unsaturated ternary and quaternary systems by a modified form of the Zdanovskii-Stokes-Robinson (ZSR) model or Wang's partial ideal solution (PIS) model and for the NH4NO3-saturated quaternary systems by Wang's ideal-like solution (ILS) model (Ber. Bunsen-Ges. Phys. Chem. 1998, 102, 1045), which indicate the nonzero interchange energies between NH4NO3 and each rare earth nitrate and the zero interchange energies between the unlike rare earth nitrates. The ILS model is completely verified by the isopiestic osmotic coefficient measurements and by the Pitzer model calculations for the NH4NO3-saturated quaternary systems.