  The four-time correlation function of a general dynamical variable obeying
Gaussian statistics is calculated for the trap model with a Gaussian density of
states. It is argued that for energy-independent variables this function is
reminiscent of the four-time functions that have been discussed earlier in the
interpretation of the results of four-dimensional NMR experiments on
supercooled liquids. Using an approximative relation between the four-time
correlation function and the cubic response function the nonlinear
susceptibility is calculated and the results are compared with the
corresponding ones resulting from an exact calculation. It is found that the
results of the approximation change the qualitative behavior of the modulus of
the susceptibility. Whereas in the exact calculation a peak is found in the
modulus in most cases, depending on temperature and the additional model
parameters no such peak occurs in the approximation. This difference has its
origin mainly in an incorrect estimate of the static response. The results are
discussed in relation to recent experimental findings.
