  Short-range lattice superstructures have been studied with high-energy x-ray
diffuse scattering in underdoped, optimally doped, and overdoped $\rm
(Y,Ca)Ba_2 Cu_3 O_{6+x}$. A new four-unit-cell superstructure was observed in
compounds with $x\sim 0.95$. Its temperature, doping, and material dependence
was used to attribute its origin to short-range oxygen vacancy ordering, rather
than electronic instabilities in the $\rm CuO_2$ layers. No significant diffuse
scattering is observed in YBa$_2$Cu$_4$O$_{8}$. The oxygen superstructures must
be taken into account when interpreting spectral anomalies in $\rm (Y,Ca)Ba_2
Cu_3 O_{6+x}$.
