structural slab

If you use too much cement in concrete, it can adversely affect the strength and durability of the final product. The amount of cement in a concrete mix is typically expressed as a "cement to sand ratio" or a "cement to aggregates ratio". A higher-than-normal cement ratio means that there will be more cement paste surrounding the individual aggregate particles, which can make the concrete weaker and more susceptible to cracking. Conversely, using too little cement will also lead to substandard concrete; in this case, the mixture will be more likely to experience shrinkage cracking as it dries. It's therefore important to get your proportions right when making concrete, in order to ensure a high-quality final product.

No, you don't need rebar for a 4 inch slab. However, if you're planning on putting anything heavy on the slab or if it will be subject to high stress, then adding rebar is a good idea. Reinforcing the slab with rebar increases its strength and durability, making it less likely to crack under pressure. 

If you're not sure whether or not your project needs rebar, it's always best to consult with a professional engineer or contractor. They can help you assess the risks and decide whether reinforcing the slab is necessary.

The answer to this question depends on a few factors, including the type of concrete, the weather conditions during and after the pour, and the thickness of the reinforcing bars (rebar). In general, however, a minimum thickness of 4 inches is recommended.

If you are looking for a more specific answer, it is best to consult with a professional engineer.