basement bathroom reno

I was wondering if anyone could give me some advice on rough plumbing my downstairs plumbing? I want to move my existing toilet over to make room to add a shower. I've noticed that below slab my sewer pipe is cast iron which is also true of the line that runs to my toilet. I thought I would cut into my main line using a "t" or "y" fitting and run a new abs pipe to my new toilet location. I was wondering should I use 3" or 4" pipe to come up through the concrete floor. If I use 3" I noticed the flange that goes inside the pipe (so it would be flush to the finished floor) is not a large diameter opening .
Should I therefore use a 4" pipe and then the 4" flange instead.
Any help would be greatly apprecaited.

You can do it either way since the pipe is 4". If you use 4" pipe, the flange fits inside, so when you repour the concrete you broke up to move things you don't have to worry about making room for the flange to fit around the pipe as you would have if you used 3". Install the pipe long so you can cut it off once you get your finished floor on. Keep in mind that you may have to move the vent in order to make it work properly and meet code. The shower needs a 2" drain line. If you are thinking about tiling things, check out www.johnbridge.com you might also want to check out www.schluter.com and www.wedi.de about a couple of neat shower tiling systems.

Thanks for the advice! I think I've seen a 4" elbow with a reducer down to 3".
Or maybe I'll just use 4" all the way back to my main sewer line which I'm almost positive it is also 4".

If you have a 4" line, keep it 4" all the way. When you repour the concrete around a 3" pipe, you have to leave a space around it for the flange to fit around the 3" pipe. If you use 4", the flange fits inside the pipe...no gaps required. Just make sure that the pipe (regardless of the type) is coming out of the floor plumb - if it doesn't do that, getting the flange to site flat and allowing the new toilet to fit on it properly is really tough.

toilet

The make flanges that go inside and outside of 4" pipe, and even though they make one that goes into a 3" pipe, I would never use one. If you must use a 3" riser, which I also never do, then use a flange that goes on the outside of the pipe.