What concrete type should I use for a pizza oven base slab?
The slab loading (pressure over area) is trivial - you have 4 square meters to carry 1600 kilos (rounding up) which is pretty much nothing for any grade of concrete.
The more important function is spreading that load (plus the mass of the slab itself) across the soil, but that's still a very low loading (roughly 4 KPa, plus the slab (at roughly another 1000 kg, so perhaps 7 kPa total),  if I have my SI units straight) while concrete compressive strength is 15 MPa at the low end. Clay soil bearing is about 70 kPa and other soils bear more.
A 100-110mm thick slab is probably plenty thick, and gives the 50mm concrete cover your reinforcing steel should have for corrosion resistance.
What is under the slab and your local climate may add other constraints, such as footings for frost movement resistance (or not.)