A 4 inch slab is a standard thickness for non commercial grade structures.

Pouring fresh concrete over existing concrete is not unusual. The company I work with does it frequently on remodel jobs when slabs are out of level.. You have to rough up the existing slab a bit and coat it with a commercial concrete adhesive, then apply the new concrete. We have used "Ardex" with pea gravel mixed in but our thicknesses have been less than 2 inches. This is quite expensive though. Ardex is a self leveling concrete powder and is not as structurally strong as actual concrete that has rocks and sand mixed in with it.