A numerical study of the impact that manufacturing tolerances have on the performance of an InP 4 × 4 MMI working as a 90° optical hybrid is presented, including simultaneous variations of width, thickness, and refractive index over the C and L bands. Simulation results for different figures of merit, such as optical common-mode rejection ratios (CMRRs) and phase errors, are provided for both nominal and worst case scenarios. Additionally, system simulations are performed to compute imbalance-induced power penalty. Our results indicate that the combined effect of realistic foundry tolerances on device performance is significant. In particular, a fourfold reduction is predicted between nominal (≃40 nm) and worst cases (≃10 nm) when optical CMRRs and phase errors are compared against Optical Internetworking Forum specifications. By contrast, a much greater bandwidth is expected at the system level (≥ 40 nm) if a power penalty of less than 1 dB (@BER = 10-3) is to be allowed. In fact, worst case power penalties lower than 0.25 dB are predicted over the full C band, which further proves the great potential of integrated 4 × 4 MMIs as wide-bandwidth devices for mass production of coherent receivers using state-of-the-art integration technologies.