  We present a scheme to produce an entangled four-photon state from two pairs
of entangled two-photon states. Such entangled four-photon states are
equivalent to the quantum state of two maximally entangled spin-1 particles.
The scheme can also be generated to prepare an entangled 2N-photon state from N
pairs of entangled two-photon states. Such multi-photon states play a crucial
role in fundamental tests of quantum mechanics versus local realism and in many
quantum information and quantum computation schemes.
