If you attach strips of preformed bituminous joint filler to the exterior walls, with the tops of them at finished grade elevation, they will provide a cushion to accommodate any minor settlement of the slab (should it happen) and also give you a guide for how high to place/finish the new concrete. You don't need anything special in terms of the new concrete, just a decent 3000 PSI (or better) mix will work just fine. A 4" thick slab will require just about 1 C.Y., so you'll probably be batching it yourself. If you use a prepackaged mix, such as Quikrete 5000, about 42, 80-lb. sacks will be required. Cutting the total pour in half by using a removable form down the middle will make for easier work as well as better placement/finishing access. Don't rush the finishing operation, and pour as stiff a mix as possible, since indoor placements always require longer for the bleed water to evaporate off.