unsweating cast iron connections

I am remodling my basement bath and have broken open the floor to replace galvanized DWV lines.

What I found was:

The 2" galvanized shower and sink drains connect to a reducing nipple threaded on one side and leaded to a 4" Cast Iron Y on the other.
The toilet is a 4" lead bend also connected to the 4" Y.

Can I unsweat the lead bend and the reducing nipple, then use 4" PVC donuts? Or is there a better course of action? I could use PVC threaded adapter on the reducing nipple.

You should be able to. I find it easier to cut out the lead.

Is it hard to do? I've never done it before. Will a regular propane torch do the trick? Will I need alot of heat, pulling, twisting, banging, etc.

A regular propane torch (pencil tip) on a 4" hub will take forever and might never work The hub will draw away lots of the heat.

how about the little map gas torch ?

You need heat, not temperature, so any "little" torch isn't going to do much good. I only had to disassemble one of these joints in my life. I drilled a couple of holes in the lead and used a tool I don't remember the name of to pry the lead out of the joint. It looks like a giant corkscrew with only 1/2 turn or so -- I think it's an automotive tool, maybe used in brake service. I poked the business end in the drilled holes and worked the lead out bit by bit around the pipe. I had good access, which helped. Once you get underneath the lead it isn't bad.