  The four-point correlation function of two 1/2 BPS primaries of conformal
weight $\Delta=2$ and two 1/2-BPS primaries of conformal weight $\Delta=n$ is
calculated in the large 't Hooft, large $N$ limit. These operators are dual to
Kaluza-Klein supergravity fields $s_k$ with masses $m^2=-4$ and $m^2=n(n-4)$.
Given that the existing formalism for evaluating sums of products of SO(6)
tensors that determine the effective couplings is only suitable for primaries
with small conformal dimensions, we make us of an alternative formalism based
on harmonic polynomials introduced by Dolan and Osborn.
  We then show that the supergravity lagrangian relevant to the computation is
of sigma-model type (i.e., the four-derivative couplings vanish) and that the
final result for the connected amplitude splits into a free and an interacting
part, as expected on general grounds.
