  This work introduces a new class of PT-symmetry grating assisted devices for
switching or modulation applications. Their operation is based on a four-wave
interaction, thus marking a step in the further development of PT-symmetry
devices which currently are essentially based on two-wave interactions. A
remarkable feature of the new device is that all their properties also hold for
the case of imperfect PT-symmetry operation, corresponding to the important
practical case of fixed losses.
