Unless you are racing with slicks, or going to huge tires, the stock tire will limit the strain on the C4 and rearend.
There are many posi units available for 8 inch rear ends and better than Ford axels. A great upgrade. If your car has an open rear end, you will have a hard time breaking anything with an automatic.
The C4 in stock form will be just fine unless you really start hammering it. There are many upgrades to the C4 and due to its light weight it is the choice over a C6 to build up.
Rob