Subfloor fix and new flange?

I have to replace a flange and some 3/4 plywood around an upstairs toilet. It is 4" ABS pipe with an ABS/plastic flange. How do I replace the flange/sleeve? Can I insert a flange/insert into the existing pipe or will the smaller size "bottleneck" the toilet waste?

Thanks! Tom

Photo:
http://www.flickr.com/photos/54361298@N00/7492183782/in/photostream

Will this work? And does the new flange sit flush to the 3/4" plywood?
http://www.homedepot.com/h_d1/N-5yc1v/R-202274073/h_d2/ProductDisplay?catalogId=10053&langId=-1&keyword=toilet+flange&storeId=10051

That flange will fit into a 4" fitting or over a 3" pipe. It will NOT fit into a 4" closet bend. SOME 3" hub closet flanges are sized to fit into a 4" pipe snug enough so they can be cemented in, but NOT all will work that way.