In this study, merging two photonic crystal-based structures, a new design for an all-optical 2-to-4 decoder has been proposed. The switching operation is based on the Kerr effect and refractive index modification. The structure consists of one nonlinear ring resonator and three nonlinear cavities that have been modified for entering the slow-light regime in order to enhance coupling through waveguides. The maximum group index of 94 has been obtained for the proposed slow-light waveguides. With this approach, the maximum and minimum normalized output powers for logic 0 and 1 are 4% and 82%, respectively. The data transfer rate of the decoder is 220 GHz, and the size of the structure is 24×9.5 μm2. The maximum insertion loss and cross talk are -7.45 dB and -16.38 dB, respectively. Considering the above characteristics, the proposed decoder can be qualified as a part of optical integrated circuits.