Reducing 6 Simple Machines to 4

1. Screw
2. Pulley
3. Wheel and axle
4. Wedge
5. Lever
6. Inclined Plane

Reducing to 4:
The lever can be mixed into the same category as the inclined plane because a lever is just an inclined plane that as change its slope.  A screw is an inclined planed wrapped around a pole that will holds materials together or lifts something.