I need to replace a 4 inch lead closet waste on a second floor bathroom.
My first mistake according to the sales person at the plumbing supply was that I cut the old lead bend flush to the hub. I did not realize until it was to late that the end that went into the hub was brass and if I would have left a couple of inches I could have used a no hub adaptor to put a new 4" PVC waste line. So he told me to chisel out the rest of the pipe and hammer in a rubber like compression fitting. The only one he stocked will reduce my waste pipe diameter to 3 inches, which I understand is ok for a second floor bath. All I am supposed to do is press the pipe into the rubber compression fitting and thats it. Does anyone have a opinion on this approach? Maybe I'm old school but I'm a little nervous going to 3 inch when I had 4" in addition to using a compression type fitting.
PS : I understand that they do sell a 4" by 4" compression fitting if I wanted to call around.

The compression fitting that you refer to is called a Duel-Tite Gasket,(see image). I would shop around for a 4" one instead of choking the branch down to 3". Let me show you how they're installed. First clear and clean the cast iron hub. Then insert a 4" Duel-Tite Gasket, (see image) in the hub. Next take the piece of plastic pipe that you wish to convert to and file and bevel the sharp edges so it will slip into the gasket.
Next you will need Duel-Tite Lubricant to make the pipe slde in without friction. Apply the lubricant to both the pipe and the gasket. Now force the pipe into the gasket, KEEP IT SQUARE, do not cock it.
If you have a problem getting it started it's acceptable to take a razor knife and shave down the first few rows of ridges. You have just converted from cast iron to plastic. Good luck, Tom