A monolithic pour means that the footers and the floor have been poured as one, are you sure first off that is is a monolithic pour as the floor is normally poured 2-4" above the footer, 4" is better but sometimes not feasible. The re-bar drilled into the footer now, because that is what you have now, and you probably don't have the space to put 4" on top is fine, he is also correct in saying that he can skim coat the remaining footer but it will crack up unless he uses a concrete glue that makes it adhere to the concrete. The re-bar that he is going to put in keeps the floor from shifting and settling. You didn't hurt the integrity of your house by taking up the slab, cracks in concrete come from settling, and not having expansion joint at stress points. You will always get them in inside corners and will usually go to lolly columns or structural post. Hope that some of this is help full to you. Everyone may have a different solution and some will make you feel that you have a real problem and charge you more that what the job really is. 4000 psi is great and no you don't need wire mesh or re-bar if you feel more comfortable you may ask for it but not necessary, you may also get fiber in you concrete which makes it stronger. Do you have about 4" of stone in what you jackhammered out?, and is this all that you are repairing, what you have jackhammered out or are you doing the whole floor?