How best to repair 24 sqft dirt hole in concrete slab
Probably low-stress, may not need anything much, but in the "better safe than sorry" mode I'd drill half-inch holes around the edges and stick reinforcing rod into them anyway. Steel is cheap compared to the rest of the job (labor and concrete cost.) That also deals with the "lock in" aspect better than chiseling.

You won't change the drainage under the rest of the slab by adding stone here, so don't bother. 

You won't change the thickness of the rest of the slab either, so unless you have a heavy machine you're mounting on this particular spot that needs a sturdy foundation, 4" should be plenty.