Toilets, Sinks, Showers, Tubs and Disposals - Connecting 2'' pvc to 4'' pvc

By way of history. I took a tub out to be replaced with a shower. The tub had 1 1/2 inch pcv piping from the drain. I have been told that code may require 2 inch pvc. I took up some subfloor and traced the drain pipe to where it ties into the 4 inch. It is about 2 1/2 feet away.

How the question. Can I put in 2 inch pipe from the shower reducing it to 1 1/2 at the junction of the 4 inch pipe so I dont have to cut into the 4 inch and replace the splitter. (I don't know if splitter is the correct term. I am referring to the piece you put into a running pipe so a another pipe can shoot off ot it) The one there now has a 1 1/2 inch "bud" coming off of it. Do I have to replace it with one that has a 2 inch "bud" coming off or can I just reduce the 2 inch to 1 1/2 inch thus using the existing splitter?

Thanks

The drain has to be the same size ALL the way to the main. This means you will have to replace the 4x1-1/2" reducer with a 4x2". Good luck.