We present a design of an all-optical 4 to 1 multiplexer by cascading four stages of ψ-shaped three-input AND gates and a Y-shaped four-input OR gate on a square lattice photonic crystal with rods in air interface. The three-input AND gate was designed separately to achieve efficient logical output and integrated into the multiplexer, and the OR gate design was subsequently incorporated with the multiplexer design. The band diagram of the proposed structures was computed using the plane wave expansion method, and the electric field distributions were estimated using the finite-difference time-domain method. The all-optical three-input AND gate had a normalized intensity of 90% for logic “1” and 10% for logic “0,” a contrast ratio (CR) of 9.54 dB, a response time of 0.816 ps, and an operating bit rate of 1.224  Tb  /  s with a footprint of 189  μm². The all-optical 4 to 1 multiplexer had a normalized intensity of 79% for logic “1” and 2% for logic “0,” a CR of 15.96 dB, a response time of 0.848 ps, and an operating bit rate of 1.179  Tb  /  s with a footprint of 1040  μm². Therefore, the proposed 4 to 1 multiplexer design would be a desirable structure for optical signal processing and photonic integrated circuits.