This study aims to investigate the impact of a short implant placed behind the mental foramen with the all-on-four treatment concept using the three-dimensional (3D) finite element stress analysis (FEA). Six different finite element analysis models were designed according to tilted implant angle (17° and 30°), presence of short implants, and short implant diameter (4.1 mm and 4.8 mm). A 100 N force was applied vertically from the central fossa of the lower right second premolar tooth. Maximum equivalent (von Mises) and Minimum/Maximum principal (Pmin/Pmax) stress values and distributions were evaluated by 3D-FEA. The highest stress value among tilted implants was in the T17 group. T30 was the group that caused the most stress in the cortical bone. Adding the short implant to the all-on-four design reduced von Mises stress on multi-unit abutments, abutment screws, and tilted implants in both the 17° and 30° groups. At the same time, it reduced Pmin/Pmax stresses in cortical bone. Similar behavior was observed in terms of stress values and distributions for the 4.1 and 4.8 mm short implant groups. The results show that short implant placement in the posterior region in the all-on-four concept reduces stress on the bone, implants, and prosthetic parts, regardless of the diameter of the short implant. In cases where biomechanical risks such as parafunctional habits and poor bone quality increase, we recommend increasing the number of implants with short implants.