MoNbTaW refractory high entropy alloy (RHEA) powders were successfully prepared by pressureless spark plasma sintering and ball milling from pure elemental powders of Mo, Nb, Ta, and W. The sintering behavior, crystal structure and phase evolution of the RHEA were studied by SEM, XRD, and TEM. MoNbTaW RHEAs with a single BCC phase are obtained at a sintering temperature far below melting points of the elements. The formation of MoNbTaW RHEA is a dynamic process through which a continuous solid solution forms due to atomic diffusion. The elemental atoms migrate and dissolve in a sequence of Mo→Nb→Ta→W. The elements with low cohesive energy diffuse and dissolve prior to those with high cohesive energy at the same temperature. A TFDC model is also proposed to calculate the lattice constants of MoNbTaW RHAEs. In comparison, the lattice constants of MoNbTaW RHEAs calculated by the TFDC model are more accurate than those calculated by Vegard’s law. The results provide a novel method for the preparation of RHEA powders and an accurate method to calculate lattice constants of HEAs.