I need to cut away a section of the 4" vertical stack pipe in my 1917 house to add a 2" PVC line from a new shower. Trouble is that I've never seen this kind of pipe before. Most of the house is cast iron but this 6' or so section of the stack is a dark gray nubby rubberized material. Sounds like metal underneath when I bang on it but I'm not sure. Anyone know what this is? Can I replace part of it with PVC and tie it together with 2 Ferncos?

A picture would help, its likely some version of a coupler. Do not cut on that stack unless you add proper jacking. The stack can weigh several hundred lbs and come crashing down on your head.

Sure, cut it out and replace it with PVC or ABS. Home Depot carries both.

But pay attention to what Ballengerb1 advised in his post above: make sure you support your C.I. pipes before cutting.

Sure, cut it out and replace it with PVC or ABS. Home Depot carries both.

But pay attention to what Ballengerb1 advised in his post above: make sure you support your C.I. pipes before cutting.

And how you support the stack is with a raiser clamp, (see image)over your cut and either a pair of bumper jacks or 2 X 4's cut to size to brace the clamp. Do not skimp on the supports. Cast iron is very heavy and if it drops it will affect every fixture above that drains into it. Good luck, Tom