 Solution

Four distinct points in a conformal field theory with $d>2$ possess two independent conformal cross-ratios, conventionallyTranslations, rotations, dilations, and special conformal transformations can map three points to canonical positions $0$, a point on one chosen axis, and the point at infinity in a conformal completion. The fourth point retains one radial coordinate and one angle relative to that axis. Those two quantities are equivalent to $u$ and $v$; the stabilizer rotations remove all its remaining angular coordinates.

For identical scalar primary operators of dimension $\Delta$, conformal covariance fixes a kinematic prefactor and leaves an arbitrary function $\mathcal{G}(u,v)$:for one common convention. Since a generic conformal field theory has unrestricted dynamical dependence on both cross-ratios, no lower-dimensional set determines the function. Therefore