How do you tap a 2 inch drain into a existing 4 inch cast iron pipe?

They have a special cutter which you could probably rent to cut the cast iron pipe with. It would be best to go ahead and buy the tee fitting, and couplers first, that way you will know how much of a section will need to be cut from the pipe. You can keep it all cast, or use a PCV TEE fiting and neoprene couplers/stanless steel cuffs/clamps.