I want to to tie into a 4" vertical stack it has a clean out with 4" brass plug. My question is this; Is there some way to install an adapter to this cleanout using an adapter of some sort for PVC?

Hi Noeyv

This is a practice that is not only against code, but also seriously frowned upon as the cleanout has no direction and the threads of the cleanout tend to catch crap/stuff and clog up the drains pretty easily.

However, with that being said, some people will still do it if they can find a proper transition bushing and they aren't going to have the work inspected.

Measure across the cap... what is its diameter?

I would prefer to see you cut out the cleanout and install a new drain fitting and a cleanout above that... ;)

Mark