My cast iron toilet flange is to high and the new toilet will rock unless I shim it like the old one was,
which I don't want to do. My question is would it be better to break the old flange out and use a 4" grinder
to grind it off a little lower and replace it with a pvc flange made to go into cast iron. Or can I use a cast iron
cutter go in the basement cut the cast iron pipe a foot below the floor and replace it with pvc and a compression coupler.
Or is their a better way to do it.

either way will work. less mess upstairs if you cut the pipe below the floor.
make sure to use a flange with a stainless steel ring secured well to the floor and solid brass bolts.

I am the wrong person to ask, because all of those methods are more difficult than necessary in my opinion. I would just break the old flange off, install a new cast iron flange and then shorten the pipe, if necessary.

hj, we did that all the time when installing cast pipe and a poured flange. we never cut the pipe until after the joint was poured and then we cut it with a hammer. stuff a rag into the pipe and tap the excess outward just above the lead. didn't get a nice clean cut but it did "cut"..

It's the only way I ever did it also.