I want to add a shower/tub to my 1/2 bath to make it a full bath I noticed that there is an extra 4" cast iron drain flush with the slab that would be about where the tub drain would go my question is how do I go about adding a trap the proper way with code and all

If we're talking about a 4" floor drain you will have to take up thje floor drain and reduce to 2". You will then install a 2"trap and raiser 15 1/2" off the back wall and 1 1/2" off the front plate to the center of the raiser. You will also need to cut a 12 X12" tub dap-out around the trap in the cement. Good luck, Tom