I have a 4" cast iron pipe that I need to move about 4 inches. It drops from my toilet about 5' into a sweep that goes vertically into a bell (6' length of cast iron). I intend on replacing the cast iron from the bell to the toilet with PVC schedule 40. My question is should I use a donut into the bell or should I cut the cast iron pipe past the bell and use a coupler I can do either but which would be better?

Bell is always better.

I agree with HVAC, ( welcome HVAC)

A donut will give you a stronger connection. Let me build a bit on HVAC's answer. You'll need to grease the end of the PVC and the inside of the donut before you attempt to lever the PVC into the donut. Sometimes starting the PVC can be difficult. If that happens I do two things. BIn addition to the grease I take a file and bevel the edge of the PVC and take a sharp knife and pare down the first two or three rows of ridges of the donut. This gets the PVC started in the bell and from there on it's a easy task to lever it home.
Good luck. Tom

Thanks Guys that's The Answer I was looking for