In this study, we present the design and optimization of a 2×4 quadrature phase and power splitter based on cascaded RI-MMI couplers integrated with thermo-optic phase shifters on a silicon photonic platform. The device operates under the restricted interference regime to achieve a compact and efficient configuration. The proposed circuit consists of two cascaded 2×2 RI-MMI couplers with integrated Ti-based heaters enabling precise thermo-optic phase control and switching capability. Numerical simulations using the 3D Beam Propagation Method (BPM) show that the splitter performs effectively across a broad wavelength range of 1.5–1.6 µm, with an excess loss of approximately 0.2 dB at the central wavelength. The four output ports exhibit well-balanced power distribution, with a maximum deviation of about 15 dB, and maintain stable 90° phase differences among the outputs. The device also demonstrates robust thermal stability and tolerance to fabrication errors up to ±1 nm, ensuring consistent optical performance. These findings highlight the strong potential of the proposed device for advanced optical signal processing applications such as QPSK modulators, multicarrier phase-coded OFDM systems, optical phasors, and multiphase local oscillators, providing a robust and high-precision platform that operates reliably across multiple wavelength bands.