The silicon crystal WASO04 is a reference in the adjustment of fundamental physical constants, but its lattice parameter has never been measured in absolute terms. In the framework of an international project meant to base the kilogram definition on the molar volume and the lattice parameter of 28Si, the WASO04 crystal has been used to manufacture an interferometer prototype for the performance testing and the fine-tuning of a new experimental apparatus for lattice parameter measurements by combined x-ray and optical interferometry. The present paper discusses the test results and gives an accurate lattice parameter determination. With respect to previous determinations, the value obtained, d220(WASO04)=192.015 570 2(10) pm, displays a four-fold improvement in accurracy.