Reinforcing bars are not required for every concrete project. The natural strength of concrete will be able to withstand the pressures to be applied to it, and the floor will help hold it together. No, you don't need rebar for a 4-inch above grade concrete slab. A 4-inch thick cast slab on the ground and in permanent contact with it will float and no rebar is required.
It is recommended to use reinforcing bars in concrete 5 to 6 inches thick. The purpose of the concrete reinforcing bar is to provide the concrete with greater tensile strength to minimize cracking. Concrete slabs poured into the ground with a properly prepared and compacted base, and which are not expected to withstand heavy loads, do not require reinforcing bars. Neither reinforcing bars nor meshes rust inside concrete as one might suspect.
Curing concrete isolates it from oxygen, which is what causes steel to oxidize. That's why thicker concrete slabs are better suited to steel reinforcement. This thin level of concrete cannot be achieved when exposed to the elements or in contact with the ground. Generally speaking, if you are making flat concrete slabs in the ground and no more than four to five inches thick, you don't need rebar.
Rebar concrete is a (relatively) new approach with completely new structural strengths than just poured and dried concrete. In general, when laying flat sheets of concrete, you should make sure that the concrete slab does not float when it rains because it acts like a boat in the mud. At the same time, it prevents cracks from traveling along a concrete slab, helping concrete withstand pressures it would have failed before. Reinforcing steel is used to reinforce concrete and reduce the chances of it cracking and breaking.
When the concrete dries, this tension is released, further compressing the concrete, which helps to apply the tensile strength of the reinforcing bar to the concrete itself. If concrete needs to support heavy machinery, heavy vehicles, a hot tub, a spa, or a concrete pizza oven, it is recommended to use rebar. For cast-in-place concrete that will be in permanent contact with the ground, the recommended thickness of cover concrete is 3 inches. Contact with the ground or the outside climate can increase the required depth of the concrete covering the reinforcement.
Rust marks on the surface of the concrete, unless it has been left by something placed on the concrete, often indicate oxidation of the reinforcing steel. Depending on the type of rebar you use, how well the concrete is sealed, and how much moisture is introduced, reinforced concrete can only last less than a hundred years. The American Concrete Institute (ACI) lists the factors that influence the thickness of the facing concrete to support the rebar.