Slot Cutting in a round hole.

I think I'd try putting a plug in the hole and use a hollow chisel mortiser to cut the slot. The plug would be there to support the chisel on the open side of the slot.

John

Well, you could use a piece of bandsaw blade in a frame saw (kind of like a hacksaw, only you make a wood frame any size you want, with a tensioner and strut instead of a stiff back). That would let you slice the sides of the slot, then it would be easy to hog out the waste with a chisel. If you really want a full four inch depth, most mortisers won't reach. The frame saw would easily do full depth, but isn't as accurate on cut path.