Base preparation is important, too. Remove all organic materials (topsoil, roots, grass) and clay soils from where you're about to place the concrete, leaving a sand and gravel base (which allows water to drain). Wet down the base and use a vibratory plate compactor or roller to compact the base.
I agree a 3000 psi mix is sufficient; as mentioned, the workability is necessary for this type of application. I do feel that a 6" thickness (with welded wire mesh) is better suited for a vehicle parking area, 4" (unreinforced) for sidewalks.
You should order only as much concrete as you can place within 90 minutes of mixing (at the plant). Concrete hardens by a chemical process called hydration, a reaction started by the addition of water, rather than "drying" - that's why you cannot just add more water to concrete as it hardens without reducing its strength.
When the concrete has cured after finishing (3-4 hours), and the surface starts to dry off, keep the surface moist for the next 4 to 5 days. Some apply liquid sealers, others use plastic or wet burlap, but the easiest way is to use a sprinkler or hose. The driveway will be ok to park on in 5 days.