We investigate the precision of Rayleigh interferometry in evaluating the nine mutual diffusion coefficients D-ij defining a four-component system by investigating the system lysozyme-tetra(ethylene glycol)-NaCl-H2O. We believe our results to be the most successful experimental determination of diffusion coefficients in quaternary systems. This choice of system is motivated by its relevance to protein crystal growth. The comparison of the D-ij coefficients of the quaternary system and the D-ij coefficients from the corresponding ternary and binary systems enables us to extract some information about the hydrodynamics and thermodynamics in solutions where protein crystals grow.