I am doing a bath remodel on a 1960s era home and an looking to connect my new PVC waste pipes into the existing cast iron pipes. I initially thought the cast iron was a 4" diameter pipe, and thus attempted to connect 4" PVC with a 4" to 4" shielded coupling and then a 4" to 4" inch fernco coupling. The connection with the first shielded coupling felt forced, and the connection with the Fernco coupling felt loose. I then realized that the cast iron pipe is 3-1/2 inches, with a 4-1/4" outside diameter. I haven't been able to find much information on this pipe size, or what type of connection I should use. I was wondering if there were any suggestions on a product that accommodates this transition or a way to get a more secure transition. Thanks