Polymethylmethacrylate (PMMA) resin is widely used in dental clinics as a matrix for provisional fixed partial dentures (PFPDs). However, PFPDs using traditional PMMA resin usually experience fractures due to low material strength. Fractures occur in the dental bridge connector after long-term wear or long-span provisional restoration. This study investigates the biomechanics of fibers embedded in a 4-unit bridge constructed from the first premolar to the second molar over the posterior mandible region. The fracture pattern and crack positions in the 4-unit bridge are evaluated. The biomechanics of the 4-unit bridge with various loading angle (45°, 90°), fiber thicknesses (0.1, 0.3, and 0.5 mm), and fiber lengths (4, 18, and 30 mm) are investigated. Reverse engineering is used to construct the PFPD model, which is imported into a finite element analysis package for biomechanical simulations. The model is experimentally validated. Statistical analysis is employed for data interpretation. The results reveal that the maximum principal stress is located below the connector of the 4-unit bridge. This stress pattern is confirmed by experimental results. The loading angle is the most important PFPD factor. A loading angle of 45° has more impact than that of 90°. For a loading angle of 45°, the percentage contributions of the fiber thickness and length are 58% and 34%, respectively, indicating that the former is a more important factor for resin strength reinforcement. For a loading angle of 90°, the fiber thickness and length have percentage contributions of 50% and 48%, respectively, indicating that the two parameters have roughly equal influence on the resin strength reinforcement. Furthermore, the bridge reinforcement enhancement obtained by increasing the fiber length from 4 mm to 18 mm is higher than that obtained by increasing the length from 18 mm to 30 mm. These results can be used by clinicians to design PFPDs that avoid lateral movement interference. The results suggest that thicker fiber or fiber longer than 18 mm should be used to improve reinforcement.