This weekend I want to remove a redundant length of (nearly horizontal) 4.5"
iron soil pipe. It joins the stack into an iron socket.

I anticipate I won't be able to remove the pipe from the socket cleanly, and
expect to need to use a disc cutter to tidy up the end once I've removed the
pipe (probably via judicious use of a sledge hammer to encourage brittle
fracture).

Is there a preferred technique for sealing the resulting hole?