Rebuilding a deck that's falling apart?

The first thing you need to do, is contact your local building inspection office, to see if you need a permit - if you do, they will have a minimum code requirement you will have to meet.

As far as replacing the 4x4's themselves, if all you wanted to do was replace them individually, you'd have to provide temporary supports to the deck, while the existing lumber is being removed. OVERBUILD the temporary support! You may have to dig to provide a concrete base for the post in the brace. I'd cut off and remove the post that is set into the concrete deck - if it's rotted, this will be fairly easy to do. You can use a jackhammer, or concrete saw to remove the portion of the concrete deck holding that post, and pour new concrete for the new post. If the post itself is really rotten, it might be easier to dig out the remainder of the wood post from the concrete, and then just jackhammer the remaining 'socket' to accept the new post and concrete.

Have Fun

If you paid the money to the contractor and signed a contract with the contractor then the contractor is liable. It is not your fault that the contractor hired a shotty sub. In some states you can file a grievance against the contractor thru the state contracting board of registrar regulatory agency for the licensing and oversight of contractors. Good Luck.