You will need a chain cutter for the cast iron pipe. If you arnt hurting for space near that stack and you plan on doing this yourself you might consider this next option. Get a syteline torch and melt the lead out around the clean out. Once that hub is cleared use a tye seal gasket and install a plastic 4 x (whatever size branch you need) wye. The 4" end of you wye would become your new clean out and you are free to do what you want with the branch. If that will not work for space needs then you will have to cut. A few things to remember while cutting, the pipe does not always cut cleanly so have a back up plan just in case that pipe breaks in a bad way, and support that stack very well. If you cut and dont support it, it can shift or drop and can cause leaks all over the place. Hope this helps.
Is that an ABS pipe someone has tapped into the existing cast iron wye????