People have different ways to describe Stage 1 vs Stage 2 vs Stage 4. Below is Harley SE kits available for a stock 96 ci engine for comparison purposes. I have highlighted the additional mods as you move down the kits.
This Stage 1 Big Bore Kit lets you increase the displacement of your Twin Cam 96™-equipped model from 96 cubic inches (1584cc) to 103 cubic inches (1690cc).
Includes Big Bore cylinders, Big Bore flat top piston assemblies and a Stage I Air Cleaner Kit with one-piece back plate, integral breathers, all mounting hardware, and gaskets
High-performance clutch spring is included.
The Stage II Big Bore Kit provides the parts you need to increase the horsepower of your EFI-equipped model. The 50-state street legal kit increases the displacement of your Twin Cam-equipped model from 96 cubic inches (1584cc) to 103 cubic inches (1690cc).
Includes Stage II (SE-255) cams, Big Bore cylinders, Big Bore flat top pistons, clips, and a Stage I Air Cleaner Kit with one-piece back plate, integral breathers, a high-performance clutch spring, all mounting hardware, and gaskets
Stage 4 Kit includes Screamin' Eagle® CNC Ported Factory Cylinder heads, 58mm ETC throttle body, SE-259E cams, 10.5 to 1 compression pistons, Perfect-Fit pushrods, SE clutch spring, and all required engine gaskets
Street Performance kit produces 103 hp and 110 ft-lbs of torque
All EFI-equipped models require ECM calibration (priced separately)