The D4c diffractometer, whose design and prototype stages have been successfully completed, is currently under construction. The D4c project involves upgrades in detection, neutron shielding, detector collimation and electronics. Large gains in counting rate (factor of 10) and in detector stability (factor of 5) will extend the range of high-precision isotopic substitution experiments used to obtain partial structure factors for liquids and glasses. The D4c instrument is scheduled to be commissioned in early 2000. (C) 2000 Elsevier Science B.V. All rights reserved.