  A four-time correlation function was calculated using a computer simulation
of a binary Lennard-Jones mixture. The information content of the four-time
correlation function is similar to that of four-time correlation functions
measured in NMR experiments. The correlation function selects a sub-ensemble
and analyzes its dynamics after some waiting time. The lifetime of the
sub-ensemble selected by the four-time correlation function is calculated, and
compared to the lifetimes of slow sub-ensembles selected using two different
definitions of mobility, and to the $\alpha$ relaxation time.
