  We present an efficient scheme for preparing and purifying of four-photon
Greenberger-Horne-Zeilinger (GHZ) state based on linear optics and
postselection. First, we describe how to create a four-photon GHZ state in both
polarization and spatial degrees of freedom from two pairs. Moreover, in the
presence of depolarization noise our scheme is capable of purifying the desired
state. In the regime of weak nonlinearity we design an indirect photon
number-resolving detection to distinguish two states of the two pairs. At last,
a fourfold coincidence detector click indicates creating of a
polarization-entangled four-photon GHZ state.
