How many 50lb bags for a small platform, 8' x 8' by 4" deep? And do I pour all the mix into the form and then add water?

For slab work, Fast-Setting Concrete Mix must be mixed with water prior to placement. Each 50-pound bag of QUIKRETE Fast-Setting Concrete Mix yields approximately 0.375 cubic feet when mixed. For a slab measuring 8' square at 4" thick, you will need 21-1/3 cubic feet of concrete, or 57 50-pound bags. Yields will vary with waste, uneven subgrade, etc.