A simple model of non-Markovian molecular reorientation is applied to multidimensional NMR experiments in supercooled liquids and polymers close to the glass transition. The process of exchange between components of a correlation time distribution is related with the notions "exponential/nonexponential, homogeneous/heterogeneous, Markovian/non-Markovian, and ergodic/nonergodic" often used in relation with molecular dynamics in viscous systems. By working out a detailed example of two-state exchange the difference between 2D and reduced 4D-spectra with respect to the mechanism and dynamics of molecular reorientation in complex liquids is elucidated.