  Non-Gaussianity may exist in the CDM isocurvature perturbation. We provide
general expressions for the bispectrum and trispectrum of both adiabatic and
isocurvature pertubations. We apply our result to the QCD axion case, and found
a consistency relation between the coefficients of the bispectrum and
trispectrum : tau_{NL}^(iso)~10^3 [f_{NL}^(iso)]^{4/3}, if the axion is
dominantly produced by quantum fluctuation. Thus future observations of the
trispectrum, as well as the bispectrum, will be important for understanding the
origin of the CDM and baryon asymmetry.
