Call for an outside salesman at your local readymix supplier. Tell them you would really like an on site consultation.
If you bond to the existing: If there are cracks in the existing, they will transfer into the new If there is grease and contaminants in the existing they will interfere with the bond. If the existing suffers from ups and downs they may well crack the new.
If you isolate with a layer of sand of similar: If you plan a finish other than exposed concrete, use a vapor barrier. This will force all water of hydration to migrate up, make sure the finishers understand NO finishing until the bleed water has come up. It makes a longer day. Plan on contraction joints that never allow the concrete to exceed
12 feet in any one direction. If they are saw joints make sure they are cut the same day, as soon as the cut does not ravel. Mesh or hog wire would be appropriate, fiber is considered the equivalent of mesh. Plasticizer will make the finishers' lives easier, make sure they know how to work with it.
Plan to cure the new no matter which method is used. If you use a chemical cure, make sure it is compatible with floor covering if it will be used. If the floor will be hard troweled, reduce or eliminate air content in the mix design.
4,000 # concrete may be overkill. If the topping slab is 4" you don't need the chip mix, but it will place and finish easier. The larger aggregate is stronger.