What is the recommended replacement method for a 4" cast iron sewer pipe whose flange completely broke off? I have seen on other posts 1) Twist-N-Set® Cast Iron Replacement Closet Flange and 2) Torque set closet flange. Any recommendations?

If the slots are broken than a flange repair kit's,(see image) your answer.
If something more then that the flange will have to be removed and a replacement installed. Let me know, Tom

Unfortunately the entire flange (the whole circle) sheared off. So I will need a replacement, but don't know which one. The remaining CI sewer pipe is pretty level (clean break) to the floor. We are the ground floor of a building, no basement, so I think its concrete.

You have two choices: you can break off the old ring completely and install new Cast Iron ring. It is bit more work but it can be done. Or, as you already mentioned, you can buy the Twist 'n' Set ring. It is easy to install and will work just fine.

Thanks. I am going to use the Twist and Set. One last question. Do I need to drill into the concrete then to secure it to the floor?

We do attach the ring into the concrete, especially the plastic rings. Takes few minutes and you will have solid place to attach your toilet into. Good luck with your project... Milo

Thanks. One last thing. I have had trouble drilling into my concrete walls. I have a 1/4 inch masonry bit. Is my 14v cordless drill just too weak. Can I get away with a higher power one or do I need a whole different type of drill? Thanks for all the help.

You need a drill that has Hammer Action setting. With this feature drilling into concrete walls is easy, clean and fast. Of course, a masonry bit is a must.

Thanks!