  We describe the design, construction and performance of a 4-circle in-vacuum
diffractometer for resonant elastic soft x-ray scattering and reflectivity. The
diffractometer, installed on the REIXS beamline at the Canadian Light Source,
includes 9 in-vacuum motions driven by in-vacuum stepper motors and operates in
ultra-high vacuum at base pressure of 2 x10^-10 Torr. Cooling to a base
temperature of 18 K is provided with a closed-cycle cryostat. The
diffractometer includes a choice of 3 photon detectors: a photodiode, a
channeltron and a 2D sensitive channelplate detector. Along with variable slit
and filter options, these detectors are suitable for studying a wide range of
phenomena having both weak and strong diffraction signals. Example measurements
of diffraction and reflectivity in Nd-doped (La,Sr)_2CuO_4 and thin film
(Ga,Mn)As are shown.
