floor slab

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.

Adding cement to concrete does indeed make it stronger. cement is a hydraulic binder, meaning it binds well with water. This makes it ideal for use in concrete, as the water helps to activate the chemical process that makes concrete harden. The more cement you add, the stronger your concrete will be. 

That being said, there is such a thing as too much cement. Too much cement can actually weaken concrete, as it causes the paste to become too fluid and accelerates the hydration process, leading to cracks and shrinkage. So while adding cement will make your concrete stronger, be sure not to add too much or you could do more harm than good!

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.