  We study resonance flourescence in a four level ladder system and illustrate
some novel features due to quantum interference and atomic coherence effects.
We find that under three photon resonant conditions, in some region of the
parameter space of the rabi frequencies $\Omega_1,\Omega_2,\Omega_3$, emission
is dominantly by the level 4 at the line center even though there is an almost
equal distribution of populations in all the levels. As one increases
$\Omega_3$ with $\Omega_1 and \Omega_2$ held fixed, the four level system
'dynamically collapses' to a two level system. The steady state populations and
the the resonance flourescence from all the levels provide adequate evidence to
this effect.
