how can I check to see if a 4" slab of concrete was actually poured?
If you are only interested in thickness of your concrete slab there is a method to measure it. Sometimes when someone doesn't know the strength of some old concrete element which was poured by someone else people hire specialist for this; they use special tool which is cylinder shaped and which drills the concrete and extracts concrete cylinder, which than you can measure. Don’t be afraid that you will damage the slab because it is only inch or two wide. On the other hand if you are wondering what is wrong and how to fix it you need to provide us with more details. People who wrote comments have asked the right questions, especially Ecnerwal.