  Double-pair emission from type-II parametric down conversion results in a
highly entangled 4-photon state. Due to interference, which is similar to
bunching from thermal emission, this state is not simply a product of two
pairs. The observation of this state can be achieved by splitting the two
emission modes at beam splitters and subsequent detection of a photon in each
output. Here we describe the features of this state and give a Bell theorem for
a 4-photon test of local realistic hidden variable theories.
