The solubility of the quaternary system RbCl-NdCl3-HCl(13.26 %)-H2O at 298.15 K was determined, and the corresponding equilibrium diagram was constructed. The quaternary system consists of three equilibrium solid phases, RbCl, RbNdCl4 center dot 4H(2)O (1:1 type), and NdCl3 center dot 6H(2)O. The new solid phase compound RbNdCl4 center dot 4H(2)O was found to be congruently soluble in the system. The compound RbNdCl4 center dot 4H(2)O was characterized by X-ray diffraction, X-ray diffraction single crystal structure analysis, thermogravimetry, and differential thermogravimetry. It belongs to an orthorhombic system, with space group P21212, a = 0.6986(6) nm, b = 1.1319(1) nm, c = 0.6665(6) nm, De = 2.795 g . cm(-3), and Z = 2. The compound loses its crystal water by one step at (370 to 474) K. The standard molar enthalpy of solution of RbNdCl4 center dot 4H(2)O in deionized water was measured to be -(26.97 +/- 0.24) kJ.mol(-1) by heat conduction microcalorimetry. Its standard molar enthalpy of formation was calculated to. be -(2732.3 +/- 1.0) kJ.mol(-1).