Abstract: [Objective] Mid-to-deep shale gas reservoirs exhibit a fracture–matrix composite structure, where internal weak planes play an essential role in stress evolution and fracture propagation. Few studies have simultaneously treated natural fractures as both hydraulic and mechanical weak planes, nor has there been systematic and quantitative analysis of their impacts on four-dimensional stress evolution and infill-well fracture propagation during production. [Methods] To address this gap, this study conducts laboratory tests to obtain the normal stiffness and hydraulic properties of weak planes, and develops a four-dimensional stress evolution model for mid- to deep-shale gas reservoirs that captures the coupled hydraulic–mechanical weakening behavior of natural fractures. The model is then used to analyze how weak planes perturb the in-situ stress field and the morphology of infill-well hydraulic fractures at different stages of production. [Results] Results indicate that low-stiffness weak planes are prone to deformation, with reduced internal stress and stress concentration at fracture tips. Moreover, the disturbance of the maximum horizontal principal stress increases progressively with the growing angle between the weak plane and the principal stress direction, while the minimum horizontal principal stress exhibits a non-monotonic response, first decreasing and then increasing. During production, the deviation of stress orientation is more pronounced when mechanical weak planes are considered. Correspondingly, infill well fractures extend further along the original maximum horizontal stress direction when not in contact with fracture zones, while the lateral expansion will be enhanced and the propagation along the original maximum horizontal stress is shortened. These differences remain relatively unchanged over time, reflecting the dominant influence of weak planes on stress disturbance is in the early stages and becoming stable in later stages. [Conclusion] This study reveals the disturbance of weak planes on four-dimensional stress evolution and provides theoretical guidance and practical reference for stress management and fracture optimization in mid-to-deep shale gas hydraulic fracturing and infill development.