As said earlier, I prefer 6" but many residential driveways and garage floors are poured with 4" thick and 4000 pound concrete mix. Figure about $2.50 per sq. ft. (plus/minus) for a turnkey job. They may not hold up under heavy vehicles and if a heavy delivery truck pulls in your driveway. You could go with 5" thick and use the same matting (heavy remesh in flat sections and not in a roll) used for street construction. You could also just go with thicker concrete in the parking areas. Because clay heaves so bad when it gets wet, use at least close to the same thickness of concrete for a gravel/sand subgrade.