With scale pits, we saw cut the slab into a checker board, pick the pieces out (no jack hammering required beyond the first couple pieces if you do it right), excavate down, and prep the base. I've never tried to gouge out concrete. Too much wear on the equipment, too many questions about weakening the slab, too labor intensive, I know I'd run into rebar, etc. If it was just a shallow drainage channel around a machine, sure I'd do it. But to gouge out 4 inches from a 6 ft by 6 ft square, I'd never do that. Doing it properly (saw cut, excavate, base, pour) will leave you with a properly reinforced cut out in your slab.
Also, with scale pits, there is a metal lip going all the way around the edge. That metal lip doesn't get bolted onto the cut edge of the original slab. We make the hole wider, drill and install rebar pins in the face of the cut, install the rebar grid for the new concrete, and lay the metal edging in. That metal edge becomes our pour stop.