I recently cut mine with an angle grinder/diamond blade. It was difficult to manuver and a little tricky to get at the bottom. I am thinking that someone makes a stiff carbide sawzall blade that may work easier.
Repair with the appropriate sized rubber coupling/stainless steel hoseclamp thingy. The OD of the 4" concrete pipe is much larger than that of the 4" PVC so the rubber coupling is flared out a bit. The make those couplings for several diferent types of pipe (clay for example) that are slightly different dimentions so measure accurately.
Here's a tip and old plumber at a supply house passed on to me. Use a razor knife and cut the "stop" out of the center of the rubber coupling and it will slide all the way back on to your pvc, allowing the a tight fit piece to slip into place.
Here's a tip from me. Put in cleanouts while you're at it.