Thanks jmon & Ghostmaker, you are absolutely right. The 3x4 flange slipped right in - 3.5" ID into the 4" drain pipe. Beginner stuff.
I did buy cleaner, I will use it liberally.
Question - I gouged up the inside of the 4" drain pipe a bit removing the old flange. My cuts went into the pipe maybe 1/64", and I gouged out a few fingernail diameter pieces also about 1/64" deep, about 3/4" deep into the drain pipe. Will PVC cement do a good enough job to gap those cuts? Does a gap filler/putty exist to use in PVC joints?
Does the stainless ring make that large of a difference? I currently have one without, and it has eight holes to secure it to the floor with wood screws. Does the plastic fatigue and crack over time from multiple seatings without the steel ring?
How much should the flange insert into the drain pipe as a minimum? I may have to buy a flange with a longer neck, if made. I plan to install flange on top of backer, then tile up to the flange. Looks like the flange will insert into drain pipe about 1", maybe just under.