Are #4 and M4 the same size

No, #4 and M4 screws are not the same size, though they're similar enough to cause confusion. A #4 screw has a major diameter of approximately 0.112 inches (about 2.84 mm), while an M4 metric screw has exactly 4.0 mm major diameter. The M4 is approximately 41% larger in diameter than #4, representing a significant difference that prevents any functional interchangeability.

Beyond diameter differences, the thread forms are incompatible: #4 screws use Unified Thread Standard angles and pitches (common variants include #4-40 with 40 threads per inch, or #4-48), while M4 uses ISO metric thread form with 0.7 mm standard pitch (M4×0.7) or 0.5 mm fine pitch (M4×0.5). Attempting to thread a #4 screw into an M4 tapped hole or vice versa will cross-thread and damage both the fastener and hole threads. When repairing equipment, replacing fasteners, or sourcing hardware, always verify whether specifications are metric or imperial—visual appearance alone can be misleading for similar-sized fasteners. The #4 screw is roughly comparable to M2.5 or M3 metric sizes (M2.5 = 2.5 mm, M3 = 3 mm), but even these aren't directly interchangeable due to different thread forms. For mixed-system equipment or international sourcing, maintain separate inventory of metric and imperial fasteners clearly labeled to prevent installation errors that damage threads, create weak joints, or require re-tapping holes to correct mismatched thread types.