  We explore the photon statistics of light emitted from a system comprising a
single four--level atom strongly coupled to a high-finesse optical cavity mode
which is driven by a coherent laser field. In the weak driving regime this
system is found to exhibit a photon blockade effect. For intermediate driving
strengths we find a sudden change in the photon statistics of the light emitted
from the cavity. Photon antibunching switches to photon bunching over a very
narrow range of intracavity photon number. It is proven that this sudden change
in photon statistics occurs due to the existence of robust quantum interference
of transitions between the dressed states of the atom-cavity system.
Furthermore, it is shown that the strong photon bunching is a nonclassical
effect for certain values of driving field strength, violating classical
inequalities for field correlations.
