  We present a systematic method to classify the four-level system using
$SU(4)$ symmetry as the basis group. It is shown that this symmetry allows
three dipole transitions which eventually leads to six possible configurations
of the four-level system. Using a dressed atom approach, the semi-classical
version of each configuration is exactly solved under rotating wave
approximation and the symmetry of the Rabi oscillation among various models is
studied and its implication is discussed.
