If I added a half a bath Just a toilet and shower.
I would be about 4 feet away from the main bathroom existing drain to the septic tank.
Around the clean out plug looks like a 4 pvc"pipe, I wonder what kind of pvc fitting
Would attach to the main drain, some kind of elbow or T-fitting?

You use Y and 45.

Great

Let us know how it went. Good luck with your project. Milo

Good advice. A side note, folks do additional baths all the time but many skip or forget to check the size of their tank and the builing code. How large of a tank do you have and did you pull a permit?