The potential of four-level optical phase quantizers toward coherent processing of advanced modulation formats, such as 16-QAM, is proposed and numerically demonstrated. The work shows that phase quantization achieved in fiber-based phase-sensitive amplifiers can demultiplex 16-QAM into two quadrature phase shift keying (QPSK) signals, enabling subchannel switching. The numerical study highlights the impact of the quantizer transfer function on the performance of the demultiplexing process and numerically calculates the bit error rate for each QPSK tributary after the demultiplexing procedure.