1) What's the best way to cut a 4" cast iron soil pipe that is against the wall of the house so I can't get a snap cutter chain around it?

related:

2) Is there an adapter to connect 3" pvc (toilet drain) to a 4" cast iron hub on the main stack?

Thanks

1) What's the best way to cut a 4" cast iron soil pipe that is against the wall of the house so I can't get a snap cutter chain around it?

We take enough stock out of the wall to slip a chain around the cast iron. If this is a vertical stack we ise a set of ratchet cutters to make the cut.

2) Is there an adapter to connect 3" pvc (toilet drain) to a 4" cast iron hub on the main stack?
If you have a open hub you can use a 4 X 3" Duel-Tite Gasket,
You will need Duel-Tite lubricant to coat the PVC so it will slip in and seat.
Good luck, Tom